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{{Short description|Prenatal organism between the embryonic state and birth}}
{{otheruses}}
{{about|the stage of prenatal development|other uses|Fetus (disambiguation)}}
{{redirect|Unborn child|the Seals and Crofts album|Unborn Child}} ] surrounds it.]]
{{redirect|Foetus|the band|Foetus (band)|the film|Foetus (film){{!}}''Foetus'' (film)}}
A '''fetus''' (or '''foetus''', or '''fœtus''') is a developing ] or other ] ], after the ]nic stage and before ]. The plural is '''fetuses''' ('''foetuses''', '''fœtuses''') or, very rarely, ''foeti''.
{{Use American English|date=August 2024}}
{{Human growth and development}}


A '''fetus''' or '''foetus''' ({{IPAc-en|ˈ|f|iː|t|ə|s}}; {{plural form}}: '''fetuses''', '''foetuses''', rarely '''feti''' or '''foeti''') is the unborn ] ] that develops from an ].<ref name=":0">{{Citation|last1=Ghosh|first1=Shampa|title=Fetus|date=2017|encyclopedia=Encyclopedia of Animal Cognition and Behavior|pages=1–5|publisher=Springer International Publishing|language=en|doi=10.1007/978-3-319-47829-6_62-1|isbn=9783319478296|last2=Raghunath|first2=Manchala|last3=Sinha|first3=Jitendra Kumar}}</ref> Following the ], the fetal stage of development takes place. ] is a continuum, with no clear defining feature distinguishing an embryo from a fetus. However, in general a fetus is characterized by the presence of all the major body ], though they will not yet be fully developed and functional, and some may not yet be situated in their final ].
In humans, the fetal stage of ] begins about eight weeks after ], when the major structures and ]s have formed, until birth.<ref>Some authorities suggest that the embryonic stage may last only seven weeks. See '''': "In humans, the organism is called an embryo for the first seven or eight weeks after conception, after which it is called a fetus." Also see '''', California Supreme Court: "beyond the embryonic stage of seven to eight weeks."</ref>


In human prenatal development, fetal development begins from the ninth week after ] (which is the eleventh week of ]) and continues until the ] of a ].
==Etymology and spelling variations==
The word "fetus" is from the ] ''fetus'', meaning "offspring", "bringing forth", or "hatching of young".<ref>Harper, Douglas. (2001). ''''. Retrieved ].</ref> It has ] roots related to sucking or ].<ref>'', Fourth Edition.'' Retrieved ].</ref>


== Etymology ==
''Foetus'' is an English variation on this, rather than a Latin or Greek word, but has been in use since at least 1594 according to the ], which describes "fetus" as the etymologically preferable spelling. The word "fetus" is not derived from the Latin verb ''foetare'', and therefore the superior etymological spelling does not include the letter "o".<ref>Peters, P. (1998). The Cambridge Australian English Style Guide, Cambridge: Cambridge University Press. ISBN 0-521-57634-2</ref> The variant ''foetus'' or ''fœtus'' may have originated with an error by ], in AD 620.<ref name="BMJ"> {{cite journal | last = Aronson | first = Jeff | title = When I use a word...:Oe no! | journal = British Medical Journal | volume = 315 | issue = 1 | date = July 1997 | publisher = BMJ Publishing Group Ltd
The word '']'' (plural '']'' or rarely '']''<ref name=":1">'']'', 2013, </ref>) comes from ] '']'' 'offspring, bringing forth, hatching of young'.<ref>O.E.D.2nd Ed.2005</ref><ref>Harper, Douglas. (2001). '' {{webarchive|url=https://web.archive.org/web/20130420044417/http://etymonline.com/ |date=2013-04-20 }}''. Retrieved 2007-01-20.</ref><ref>{{Cite web |url = https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0060%3Aentry%3Dfetus2 |title = Charlton T. Lewis, An Elementary Latin Dictionary, fētus |access-date = 2015-09-24 |url-status = live |archive-url = https://web.archive.org/web/20170104185503/http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0060%3Aentry%3Dfetus2 |archive-date = 2017-01-04 }}</ref> The Latin plural ''fetūs'' is ]; occasionally the plural ''feti'' is used in English by analogy with second-declension Latin nouns ending in ''-us''.<ref name=":1" />
| url = http://bmj.bmjjournals.com/cgi/content/full/315/7102/0/h | accessdate = 2006-06-29 }} </ref> The preferred spelling in the United States is ''fetus'', but the variant ''foetus'' or ''fœtus'' persists in other English-speaking countries, and in some medical contexts, as well as in some other languages (e.g. French).


The predominant British, Irish, and ] spelling is '']'', except in medical usage, where ''fetus'' is preferred. The ''-oe-'' spelling is first attested in 1594 and arose in ] by analogy with classical Latin words like ''amoenus''.<ref>''New Oxford Dictionary of English''.</ref>
==Human fetus==
]]]The fetal stage begins eight weeks after fertilization. The fetus is not as sensitive to damage from environmental exposures as the embryo was, though toxic exposures can often cause physiological abnormalities or minor congenital malformation. Fetal growth can be terminated by various factors, including ], ] committed by a third party, or induced ].


===Development=== == Development in humans ==
{{seealso|Prenatal development}} {{Further|Prenatal development}}


===Weeks 9 to 16 (2 to 3.6 months)===
The following timeline describes some of the specific changes in fetal ] and ] by fertilization age (i.e. the time elapsed since fertilization). However, it should be noted that obstetricians often use "]" which, by convention, is measured from 2 weeks earlier than fertilization. For purposes of this article, age is measured from fertilization, except as noted.
], at three months gestational age]]


In humans, the fetal stage starts nine weeks after fertilization.<ref name="nursing">Klossner, N. Jayne, (2005): "The fetal stage is from the beginning of the 9th week after fertilization and continues until birth"</ref> At this time the fetus is typically about {{convert|30|mm|in|frac=4}} in length from ], and weighs about 8&nbsp;grams.<ref name="nursing" /> The head makes up nearly half of the size of the fetus.<ref name="itrfps">{{cite web|url=https://www.nlm.nih.gov/medlineplus/ency/article/002398.htm|title=Fetal development: MedlinePlus Medical Encyclopedia|website=www.nlm.nih.gov|url-status=live|archive-url=https://web.archive.org/web/20111027150119/http://www.nlm.nih.gov/medlineplus/ency/article/002398.htm|archive-date=2011-10-27}}</ref> Breathing-like movements of the fetus are necessary for the stimulation of ], rather than for obtaining oxygen.<ref>Institute of Medicine of the National Academies, '' {{webarchive|url=https://web.archive.org/web/20110607061030/http://books.nap.edu/openbook.php?record_id=11622&page=261|date=2011-06-07}}'' (2006), page 317. Retrieved 2008-03-12</ref> The heart, hands, feet, brain, and other organs are present, but are only at the beginning of development and have minimal operation.<ref name="Columbia">'' {{webarchive|url=https://web.archive.org/web/20071012162642/http://bartleby.com/65/fe/fetus.html |date=2007-10-12 }}'' (Sixth Edition). Retrieved 2007-03-05.</ref><ref>Greenfield, Marjorie. " {{webarchive|url=https://web.archive.org/web/20070122182805/http://www.drspock.com/article/0%2C1510%2C9851%2C00.html |date=2007-01-22 }}". Retrieved 2007-01-20.</ref> Uncontrolled movements and twitches occur as ]s, the ], and pathways begin to develop.<!--"twitches reference found on pages 233, 472, 726 in Prechtl--><ref name="Prechtl">Prechtl, Heinz. in ''Handbook of brain and behaviour in human development'', Kalverboer and Gramsbergen eds., pp. 415–418 (2001 Kluwer Academic Publishers): "The first movements to occur are sideward bendings of the head.&nbsp;... At 9–10 weeks postmestrual age complex and generalized movements occur. These are the so-called general movements (Prechtl et al., 1979) and the startles. Both include the whole body, but the general movements are slower and have a complex sequence of involved body parts, while the startle is a quick, phasic movement of all limbs and trunk and neck."</ref>
; 8 weeks (condition at start of fetal stage) : The risk of ] decreases sharply at the beginning of the fetal stage.<ref> -. (August 6 , 2002). ''BBC News.'' Retrieved ].</ref> At this point, all major structures, including hands, feet, head, brain, and other organs are present, but they continue to grow, develop, and become more functional.<ref>'''' (Sixth Edition). Retrieved ].</ref> When the fetal stage commences, a fetus is typically about 30 mm (1.2 inches) in length, and the heart is beating.<ref>Greenfield, Marjorie. “". Retrieved ].</ref> The fetus bends the head, and also makes general ] and startles that involve the whole body.<ref name="Prechtl">Prechtl, Heinz. "" in ''Handbook of brain and behaviour in human development'', Kalverboer and Gramsbergen eds., pp. 415-418 (2001 Kluwer Academic Publishers). Retrieved ].</ref> ] activity has been detected as early as 54 days after conception.<ref name="Singer">Singer, Peter. '''', page 104 (St. Martins Press 1996). Retrieved ].</ref> Some fingerprint formation can be seen from the beginning of the fetal stage.<ref>Zabinski, Mark. Forensic Series Seminar, Pastore Chemical Laboratory, University of Rhode Island (February 2003) ( retrieved ]).</ref> (large image of fetus at 8 weeks after fertilization). Retrieved ]. A rotatable 3D version of this photo is available , and a sketch is available .</ref>]]
; 8 to 15 weeks : The fetus continues to move in distinct motor patterns, picking up new patterns such as localized movement of the arms and legs, hiccups, breathing-like movements, and stretches and yawns .<ref name="Prechtl"/><ref>Butterworth, George and Harris, Margaret. '''' (Psychology Press 1994).</ref> The breathing-like movement of the fetus is necessary for stimulation of lung development, rather than for obtaining oxygen.<ref>Institute of Medicine of the National Academies, '''' (2006). Retrieved ].</ref> At nine weeks, the fetus is able to bend fingers around an object; in response to a touch on the foot, the fetus will bend the legs or curl the toes to move away from the object.<ref>Valman, H. and Pearson, J. "", ''British Medical Journal'', (January 26, 1980). Retrieved ].</ref> The face is well-formed and develops a more human appearance. Eyelids close and remain closed for several months. The different appearance of the genitals in males and females becomes pronounced. ] buds appear, the ]s are long and thin, and ]s are produced in the ]. A fine hair called ] develops on the head. The gastrointestinal tract, still forming, starts to collect sloughed skin and lanugo, as well as hepatic products, forming ] (stool). Fetal ] is almost transparent. More muscle tissue and bones have developed, and the bones become harder. The first measurable signs of ] activity occur in the 12th week.<ref name="Singer"/><ref>Vogel, Friedrich. '''' (Springer 2000): "Slow EEG activity (0.5 – 2 c/s) can be demonstrated in the fetus even at the conceptual age of three months." Retrieved ].</ref> By the end of this stage, the fetus has reached about 15 cm (6 inches). (large image of fetus at 18 weeks after fertilization). Retrieved ]. A rotatable 3D version of this photo is available , and a sketch is available .</ref>]]


=== Weeks 17 to 25 (3.6 to 6.6 months)===
; 16 to 25 weeks : The ] covers the entire body. Eyebrows, eyelashes, fingernails, and toenails appear. The fetus has increased muscle development. ] (air sacs) are forming in lungs. The ] develops enough to control some body functions. The ] are now developed, though the ] sheaths in the neural portion of the auditory system will continue to develop until 18 months after birth. The respiratory system has developed to the point where gas exchange is possible. The ], the first maternally discernable fetal movements, are often felt during this period. A woman pregnant for the first time (i.e. a primiparous woman) typically feels fetal movements at about 18-19 weeks, whereas a woman who has already given birth at least two times (i.e. a multiparous woman) will typically feel movements around 16 weeks.<ref>Levene, Malcolm et al. '''' (Blackwell 2000), page 8. Retrieved ].</ref> By the end of the fifth month, the fetus is about 20 cm (8 inches). (large image of fetus at 38 weeks aftre fertilization). Retrieved ]. A rotatable 3D version of this photo is available , and a sketch is available .</ref>]]
A woman pregnant for the first time (]) typically feels ]s at about 21 weeks, whereas a woman who has given birth before will typically feel movements by 20 weeks.<ref>Levene, Malcolm et al. '' {{Webarchive|url=https://web.archive.org/web/20230408064509/https://books.google.com/books?id=FHXtDkLzOHEC&pg=RA2-PA8 |date=2023-04-08 }}'' (Blackwell 2000), p. 8. Retrieved 2007-03-04.</ref> By the end of the fifth month, the fetus is about {{convert|20|cm|in|0|abbr=on}} long.


=== Weeks 26 to 38 (6.6 to 8.6 months)===
; 26 to 38 weeks : The amount of body fat rapidly increases. Lungs are not fully mature. ] brain connections, which mediate sensory input, form. Bones are fully developed, but are still soft and pliable. ], ], and ] become more abundant. Continuous EEG readings have been observed by the 30th week.<ref name="Singer"/> Fingernails reach the end of the fingertips. The lanugo begins to disappear, until it is gone except on the upper arms and shoulders. Small ]s are present on both sexes. Head hair becomes coarse and thicker. Birth is imminent and occurs around the 38th week. The fetus is considered full-term between weeks 35 and 40,<ref> ''BabyCenter.com'' Retrieved June 1, 2007.</ref> which means that the baby is considered sufficiently developed for life outside the womb.<ref>, retrieved ].</ref> It may be 48 to 53 cm (19 to 21 inches) in length, when born.
The amount of body fat rapidly increases. Lungs are not fully mature. Neural connections between the ] and ] develop as early as 24 weeks of gestational age, but the first evidence of their function does not occur until around 30 weeks.{{citation needed|date=November 2022}} Bones are fully developed but are still soft and pliable. ], ], and ] become more abundant. Fingernails reach the end of the fingertips. The ], or fine hair, begins to disappear until it is gone except on the upper arms and shoulders. Small breast buds are present in both sexes. Head hair becomes coarse and thicker. Birth is imminent and occurs around the 38th week after fertilization. The fetus is considered full-term between weeks 37 and 40 when it is sufficiently developed for ].<ref>{{cite web|url=https://www.nhs.uk/pregnancy/week-by-week/28-to-40-plus/37-weeks/#:~:text=At%2037%20weeks%2C%20your%20pregnancy,in%20the%20next%20few%20weeks.|publisher=NHS.UK|title=You and your baby at 37 weeks pregnant|date=8 December 2020|access-date=2022-11-01|archive-date=2022-11-02|archive-url=https://web.archive.org/web/20221102014312/https://www.nhs.uk/pregnancy/week-by-week/28-to-40-plus/37-weeks/#:~:text=At%2037%20weeks%2C%20your%20pregnancy,in%20the%20next%20few%20weeks.|url-status=live}}</ref><ref>{{cite web|url=https://www.parents.com/pregnancy/giving-birth/why-it-pays-to-wait-for-a-full-term-baby-birth/|publisher=Parents|title=Giving Birth Before Your Due Date: Do All 40 Weeks Matter?|access-date=2022-11-01|archive-date=2022-11-02|archive-url=https://web.archive.org/web/20221102014312/https://www.parents.com/pregnancy/giving-birth/why-it-pays-to-wait-for-a-full-term-baby-birth/|url-status=live}}</ref> It may be {{convert|48|to|53|cm|in|0|abbr=on}} in length when born. Control of movement is limited at birth, and purposeful voluntary movements continue to develop until ].<ref>Stanley, Fiona et al. , page 48 (2000 Cambridge University Press): "Motor competence at birth is limited in the human neonate. The voluntary control of movement develops and matures during a prolonged period up to puberty...."</ref><ref name="Becher">Becher, Julie-Claire. {{cite web|url = http://pregnancyarchive.com/articles/insights-into-early-fetal-development |title = Insights into Early Fetal Development |url-status = dead |archive-url = https://web.archive.org/web/20130601182944/http://pregnancyarchive.com/articles/insights-into-early-fetal-development/ |archive-date = 2013-06-01}}, ''Behind the Medical Headlines'' (Royal College of Physicians of Edinburgh and Royal College of Physicians and Surgeons of Glasgow October 2004) <!-- archived article --></ref>


====Variation in growth==== ===Variation in growth===
{{seealso|Birth weight}} {{Further|Birth weight|Environmental toxicants and fetal development}}
There is much variation in the growth of the fetus. When fetal size is less than expected, that condition is known as intrauterine growth restriction (IUGR) also called fetal growth restriction (FGR); factors affecting fetal growth can be ''maternal'', '']l'', or ''fetal''.<ref name="Holden">Holden, Chris and MacDonald, Anita. '''' (Elsevier 2000). Retrieved ].</ref> There is much variation in the growth of the human fetus. When the fetal size is less than expected, the condition is known as ] also called fetal growth restriction; factors affecting fetal growth can be ''maternal'', '']l'', or ''fetal''.<ref name="Holden">Holden, Chris and MacDonald, Anita. '' {{Webarchive|url=https://web.archive.org/web/20200731092010/https://www.abebooks.com/9780702024214/Nutrition-Child-Health-Chris-Holden-070202421X/plp |date=2020-07-31 }}'' (Elsevier 2000). Retrieved 2007-03-04.</ref>


'''Maternal''' factors include maternal ], ], nutritional state, emotional ], toxin exposure (including ], ], ], and other drugs which can also harm the fetus in other ways), and ]. A woman's primiparity also may affect fetal weight (firstborns tend to weigh less){{Fact|date=August 2007}}. * Maternal factors include maternal ], ], nutritional state, ], ] (including ], ], ], and other drugs which can also harm the fetus in other ways), and ] ] flow.


'''Placental''' factors include size, microstructure (densities and architecture), ], transporters and binding proteins, nutrient utilization and nutrient production. * Placental factors include size, microstructure (densities and architecture), ] blood flow, transporters and binding proteins, nutrient utilization, and nutrient production.


'''Fetal''' factors include the fetus genome, nutrient production, and ] output. Also, female fetuses tend to weigh less than males, at full term.<ref name="Holden"/> * Fetal factors include the fetal genome, nutrient production, and ] output. Also, female fetuses tend to weigh less than males, at full term.<ref name="Holden"/>


Fetal growth is often classified as follows: small for gestational age (SGA), appropriate for gestational age (AGA), and large for gestational age (LGA).<ref>Queenan, John. '''' (Blackwell 1999). Retrieved ].</ref> SGA can result in low birth weight, although premature birth can also result in low birth weight. Low birth weight increases risk for perinatal mortality (] shortly after birth), ], ], ], ], ], ] abnormalities, and other long-term health problems. SGA may be associated with growth delay, or it may instead be associated with absolute stunting of growth. Fetal growth is often classified as follows: ] (SGA), ] (AGA), and ] (LGA).<ref>Queenan, John. '' {{Webarchive|url=https://web.archive.org/web/20230424003357/https://books.google.com/books?id=sfp-OzoU6X4C&pg=RA2-PA6 |date=2023-04-24 }}'' (Blackwell 1999). Retrieved 2007-03-04.</ref> SGA can result in ], although premature birth can also result in low birth weight. Low birth weight increases the risk for perinatal mortality (] shortly after birth), ], ], ], ], ], ] abnormalities, and other long-term health problems. SGA may be associated with growth delay, or it may instead be associated with absolute stunting of growth.


===Viability=== == Viability ==
{{main|Fetal viability}}
Five months is currently the lower limit of ], and viability usually occurs later.<ref>Halamek, Louis. "", ''NeoReviews'', Vol.4 No.6 (2003): "most neonatologists would agree that survival of infants younger than approximately 22 to 23 weeks’ estimated gestational age is universally dismal and that resuscitative efforts should not be undertaken when a neonate is born at this point in pregnancy."</ref> According to ''The Developing Human'':
{{wide image|Prenatal development table.svg|1200px|Stages in ], showing viability and point of 50% chance of survival at bottom. Weeks and months numbered ].}}
<blockquote>Viability is defined as the ability of fetuses to survive in the extrauterine environment... There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable or beyond which survival is assured, but experience has shown that it is rare for a baby to survive whose weight is less than 500 gm or whose fertilization age is less than 22 weeks. Even fetuses born between 26 and 28 weeks have difficulty surviving, mainly because the respiratory system and the central nervous system are not completely differentiated... If given expert postnatal care, some fetuses weighing less than 500 gm may survive; they are referred to as ''extremely low birth weight'' or ''immature infants''.... Prematurity is one of the most common causes of morbidity and prenatal death.<ref>{{cite book |author=Moore, Keith and Persaud, T. |title=The Developing Human: Clinically Oriented Embryology' |publisher=Saunders |location=Philadelphia |year=2003 |isbn=0-7216-9412-8 |pages=p. 103}}</ref></blockquote>


] refers to a point in fetal development at which the fetus may survive outside the womb. The lower limit of viability is approximately {{frac|5|3|4}} months gestational age and is usually later.<ref>Halamek, Louis. " {{webarchive|url=https://web.archive.org/web/20090608065459/http://neoreviews.aappublications.org/cgi/content/extract/4/6/e153 |date=2009-06-08 }}", ''NeoReviews'', Vol.4 No.6 (2003): "most neonatologists would agree that survival of infants younger than approximately 22 to 23 weeks' estimated gestational age is universally dismal and that resuscitative efforts should not be undertaken when a neonate is born at this point in pregnancy."</ref>
During the past several decades, expert postnatal care has improved with advances in medical science, and therefore the point of viability has moved earlier.<ref>''Roe v. Wade'', (1973) ("viability is usually placed at about seven months (28 weeks) but may occur earlier, even at 24 weeks.") Retrieved ].</ref> As of 2006, the youngest child to survive a premature birth was ] at 21 weeks and 6 days' ].<ref>Baptist Hospital of Miami, (2006).</ref>


There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable.<ref name="developinghuman">Moore, Keith and Persaud, T. , p. 103 (Saunders 2003).</ref> According to data from 2003 to 2005, survival rates are 20–35% for babies born at 23 ] ({{frac|5|3|4}} months); 50–70% at 24–25 weeks (6 – {{frac|6|1|4}} months); and >90% at 26–27 weeks ({{frac|6|1|2}} – {{frac|6|3|4}} months) and over.<ref name="marchofdimes"> {{webarchive|url=https://web.archive.org/web/20141024114718/http://www.marchofdimes.org/loss/neonatal-death.aspx |date=2014-10-24 }}, retrieved September 2, 2009.</ref> It is rare for a baby weighing less than {{convert|500|g|lboz|abbr=on}} to survive.<ref name="developinghuman"/>
===Fetal pain===
{{main|Fetal pain}}


When such ] babies are born, the main causes of ] are that neither the respiratory system nor the central nervous system are completely differentiated.<!--<ref name="developinghuman"/>--> If given expert postnatal care, some preterm babies weighing less than {{convert|500|g|lboz|abbr=on}} may survive, and are referred to as ''extremely low birth weight'' or ''immature infants''.<ref name="developinghuman"/>
The subject of fetal ] and ] is controversial. The ability of a fetus to feel pain is often part of the ]. However, according to ], "there is no consensus among the medical and scientific experts about precisely when a fetus becomes pain-capable."<ref>Caplan, Arthur. Testimony to U.S. Congress, Subcommittee on the Constitution, Civil Rights, and Civil Liberties. "." (]). Retrieved ].</ref> Different sources have estimated that the earliest point for pain sensation may be during the first 12 weeks or after 20, 24, or 26 weeks gestation, or months after birth.


] is the most common cause of infant mortality, causing almost 30 percent of neonatal deaths.<ref name="marchofdimes"/> At an occurrence rate of 5% to 18% of all deliveries,<ref name="WHO2014">{{cite web |url =https://www.who.int/mediacentre/factsheets/fs363/en/ |title = Preterm birth Fact sheet N°363 |author = World Health Organization |work = who.int |date = November 2014 |access-date = 6 Mar 2015 |url-status = live |archive-url = https://web.archive.org/web/20150307050438/http://www.who.int/mediacentre/factsheets/fs363/en/ |archive-date = 7 March 2015 |df = dmy-all }}</ref> it is also more common than ], which occurs in 3% to 12% of pregnancies.<ref>{{cite book |last1 = Buck |first1 = Germaine M. |last2 = Platt |first2 = Robert W. |title = Reproductive and perinatal epidemiology |date = 2011 |publisher = Oxford University Press |location = Oxford |isbn = 9780199857746 |page = 163 |url = https://books.google.com/books?id=by1lwSpfruQC&pg=PA163 |url-status = live |archive-url = https://web.archive.org/web/20160815200142/https://books.google.ca/books?id=by1lwSpfruQC&pg=PA163 |archive-date = 2016-08-15 }}</ref>
===Circulatory system===


== Circulatory system ==
].]]
{{main|Fetal circulation}}
The ] of a human fetus works differently from that of born humans, mainly because the lungs are not in use: the fetus obtains ] and nutrients from the woman through the ] and the ].<ref name="Whitaker">Whitaker, Kent. '''' (Delmar 2001). Retrieved ].</ref>


===Before birth===
Blood from the placenta is carried to the fetus by the ]. About half of this enters the fetal '']'' and is carried to the ], while the other half enters the ] proper from the inferior border of the liver. The branch of the umbilical vein that supplies the right lobe of the liver first joins with the ]. The blood then moves to the right atrium of the ]. In the fetus, there is an opening between the right and left atrium (the '']''), and most of the blood flows from the right into the left atrium, thus bypassing ]. The majority of blood flow is into the left ventricle from where it is pumped through the ] into the body. Some of the blood moves from the aorta through the internal iliac arteries to the umbilical arteries, and re-enters the placenta, where ] and other waste products from the fetus are taken up and enter the woman's circulation.<ref name="Whitaker"/>
]]]
The ] and ]s of the ] form relatively early during ], but continue to grow and develop in complexity in the growing fetus. A functional circulatory system is a biological necessity since mammalian tissues can not grow more than a few cell layers thick without an active blood supply. The prenatal circulation of blood is different from postnatal circulation, mainly because the lungs are not in use. The fetus obtains ] and nutrients from the mother through the ] and the ].<ref name="Whitaker">Whitaker, Kent (2001). Delmar. Retrieved 2007-03-04.</ref>


Blood from the placenta is carried to the fetus by the ]. About half of this enters the fetal '']'' and is carried to the ], while the other half enters the ] proper from the inferior border of the liver. The branch of the umbilical vein that supplies the right lobe of the liver first joins with the ]. The blood then moves to the right atrium of the ]. In the fetus, there is an opening between the right and left atrium (the '']''), and most of the blood flows from the right into the left atrium, thus bypassing ]. The majority of blood flow is into the left ventricle from where it is pumped through the ] into the body. Some of the blood moves from the aorta through the internal iliac arteries to the umbilical arteries and re-enters the placenta, where ] and other waste products from the fetus are taken up and enter the mother's circulation.<ref name="Whitaker"/>
Some of the blood from the right atrium does not enter the left atrium, but enters the right ventricle and is pumped into the ]. In the fetus, there is a special connection between the pulmonary artery and the aorta, called the '']'', which directs most of this blood away from the lungs (which aren't being used for respiration at this point as the fetus is suspended in ]).<ref name="Whitaker"/>


Some of the blood from the right atrium does not enter the left atrium, but enters the right ventricle and is pumped into the ]. In the fetus, there is a special connection between the pulmonary artery and the aorta, called the '']'', which directs most of this blood away from the lungs (which are not being used for respiration at this point as the fetus is suspended in ]).<ref name="Whitaker"/>
====Postnatal development====
{{Main|Adaptation to extrauterine life}}


<gallery class="center">
With the first breath after birth, the system changes suddenly. The pulmonary resistance is dramatically reduced ("pulmo" is from the Latin for "]"). More blood moves from the right atrium to the right ventricle and into the pulmonary arteries, and less flows through the '']'' to the left atrium. The blood from the lungs travels through the pulmonary veins to the left atrium, increasing the pressure there. The decreased right atrial pressure and the increased left atrial pressure pushes the ''septum primum'' against the ''septum secundum'', closing the ''foramen ovale'', which now becomes the ''fossa ovalis''. This completes the separation of the circulatory system into two halves, the left and the right.
File:Ultrasound_image_of_a_fetus.jpg|] of {{convert|3|in|mm|-1|adj=on|order=flip}} fetus (about {{frac|3|1|2}} months ])
File:Sucking his thumb and waving.jpg|Fetus at {{frac|4|1|4}} months
File:3dultrasound 20 weeks.jpg|Fetus at 5 months
</gallery>


===Postnatal development===
The ''ductus arteriosus'' normally closes off within one or two days of birth, leaving behind the ligamentum arteriosum. The umbilical vein and the ductus venosus closes off within two to five days after birth, leaving behind the '']'' and the '']'' of the liver respectively.
{{main|Adaptation to extrauterine life}}
With the first breath after birth, the system changes suddenly. ] resistance is reduced dramatically, prompting more blood to move into the ] from the ] and ] of the heart and less to flow through the '']'' into the ]. The blood from the lungs travels through the ]s to the left atrium, producing an increase in pressure that pushes the '']'' against the '']'', closing the ''foramen ovale'' and completing the separation of the newborn's ] into the standard left and right sides. Thereafter, the ''foramen ovale'' is known as the ''fossa ovalis''.


The '']'' normally closes within one or two days of birth, leaving the '']'', while the ] and '']'' usually closes within two to five days after birth, leaving, respectively, the liver's '']'' and '']''.
====Differences from the adult circulatory system====
Remnants of the fetal circulation can be found in adults:<ref>Dudek, Ronald and Fix, James. '''' (Lippincott 2004). Retrieved ].</ref><ref>University of Michigan Medical School, . Retrieved ].</ref>


== Immune system ==
{| class="wikitable"
! Fetal !! Adult
|-
| ] || ]
|-
|] || ]
|-
| extra-] portion of the fetal left ] || ] (the "round ligament of the liver").
|-
| intra-hepatic portion of the fetal left umbilical vein (the ]) || ]
|-
| ] portions of the fetal left and right ] || ] of the ]
|-
| ] portions of the fetal left and right umbilical arteries || ] (])
|}


The ] functions as a ] against the transmission of ]s. When this is insufficient, ] of infectious diseases can occur.
In addition to differences in circulation, the developing fetus also employs a different type of oxygen ] than adults (adults use ]). ] enhances the fetus' ability to draw oxygen from the placenta. Its association curve to oxygen is shifted to the left, meaning that it will take up oxygen at a lower concentration than adult hemoglobin will. This enables fetal hemoglobin to absorb oxygen from adult hemoglobin in the placenta, which has a lower pressure of oxygen than at the lungs.


Maternal ] cross the placenta, giving the fetus ] against those diseases for which the mother has antibodies. This transfer of antibodies in humans begins as early as the fifth month (gestational age) and certainly by the sixth month.<ref> of {{cite book |author = Pillitteri, Adele |title = Maternal and Child Health Nursing: Care of the Childbearing and Childrearing Family |publisher = Lippincott Williams & Wilkins |location = Hagerstwon, MD |year = 2009 |isbn = 978-1-58255-999-5 }}</ref>
===Developmental problems===


== Developmental problems ==
{{seealso|Congenital disorder}}
{{Further|Environmental toxicants and fetal development|Birth defect}}


A developing fetus is highly susceptible to anomalies in its growth and metabolism, increasing the risk of ]s. One area of concern is the lifestyle choices made during pregnancy.<ref>{{cite journal |last1 = Dalby |first1 = JT |year = 1978 |title = Environmental effects on prenatal development |journal = Journal of Pediatric Psychology |volume = 3 |issue = 3|pages = 105–109 |doi = 10.1093/jpepsy/3.3.105 }}</ref> Diet is especially important in the early stages of development. Studies show that supplementation of the person's diet with ] reduces the risk of ] and other ] defects. Another dietary concern is whether breakfast is eaten. Skipping breakfast could lead to extended periods of lower than normal nutrients in the maternal blood, leading to a higher risk of ], or birth defects.
Congenital anomalies are anomalies that are acquired before birth. Infants with certain congenital anomalies of the heart can survive only as long as the ductus remains open: in such cases the closure of the ductus can be delayed by the administration of ]s to permit sufficient time for the surgical correction of the anomalies. Conversely, in cases of ], where the ductus does not properly close, drugs that inhibit prostaglandin synthesis can be used to encourage its closure, so that surgery can be avoided.


Alcohol consumption may increase the risk of the development of ], a condition leading to ] in some infants.<ref>{{cite book |author = Streissguth, Ann Pytkowicz |title = Fetal alcohol syndrome: a guide for families and communities |publisher = Paul H Brookes Pub |location = Baltimore, MD |year = 1997 |isbn = 978-1-55766-283-5 |url-access = registration |url = https://archive.org/details/fetalalcoholsynd00stre }}</ref> ] may also lead to ]s and ] ({{convert|2500|g|lboz|abbr=off}}. Low birth weight is a concern for medical providers due to the tendency of these infants, described as "''premature'' by weight", to have a higher risk of secondary medical problems.
A developing fetus is highly susceptible to anomalies in its growth and metabolism, increasing the risk of birth defects. One area of concern is the mother's lifestyle choices made during pregnancy. Diet is especially important in the early stages of development. Studies show that supplementation of the mother's diet with ] reduces the risk of ] and other ] defects. Another dietary concern is the consumption of breakfast by the mother. This one factor could lead to extended periods of lower than normal nutrients in the mother's blood, leading to a higher risk of ], or other birth defects in the fetus.
During this time alcohol consumption may increase the risk of the development of ], a condition leading to ] in some infants.<ref>Streissguth, A. (1997). ''Fetal Alcohol Syndrome: A Guide for Families and Communities''. Baltimore: Brookes Publishing. ISBN 1-55766-283-5.</ref>
Smoking during pregnancy may also lead to reduced birth weight. Low birth weight is defined as 2500 grams (5.5 lb). Low birth weight is a concern for medical providers due to the tendency of these infants, described as ''premature'' by weight, to have a higher risk of secondary medical problems.


]s are known to have possible adverse effects on the development of the fetus, and the risks need to be weighed against the benefits.<ref>O'Reilly, Deirdre. " {{webarchive|url=https://web.archive.org/web/20111027150119/http://www.nlm.nih.gov/medlineplus/ency/article/002398.htm |date=2011-10-27 }}". '']'' (2007-10-19). Retrieved 2018-08-26.</ref><ref name="De Santis">{{cite journal |last1=De Santis |first1=M |last2=Cesari |first2=E |last3=Nobili |first3=E |last4=Straface |first4=G |last5=Cavaliere |first5=AF |last6=Caruso |first6=A |title=Radiation effects on development. |journal=Birth Defects Research Part C: Embryo Today: Reviews |date=September 2007 |volume=81 |issue=3 |pages=177–82 |doi=10.1002/bdrc.20099 |pmid=17963274}}</ref>
===Legal issues===
{{main|Fetal rights}}
Especially since the 1970s, there has been continuing debate over the "]" of the human fetus. Although ] of a fetus before ] is generally legal in the United States following the case of '']'', the third-party-killing of a fetus can be punishable as ] or ] throughout the pregnancy, depending on jurisdiction.


Congenital disorders are acquired before birth. Infants with certain ]s can survive only as long as the ductus remains open: in such cases the closure of the ductus can be delayed by the administration of ]s to permit sufficient time for the surgical correction of the anomalies. Conversely, in cases of ], where the ductus does not properly close, drugs that inhibit prostaglandin synthesis can be used to encourage its closure, so that surgery can be avoided.
==Non-human fetuses==
{{Expand-section|date=March 2007}}<!--Insufficient detail!-->
]]
The fetus of most mammals develops similarly to the '']'' fetus. After the first stages of development, the human embryo reaches a stage very similar to all other vertebrates<!--Technically chordates, but this is a little too much detail-->.<ref>ZFIN, . Modified from: Kimmel et al., 1995. Developmental Dynamics 203:253-310. Downloaded 5 March 2007.</ref> The anatomy of the area surrounding a fetus is different in litter-bearing animals compared to humans: each fetus is surrounded by placental ] and is lodged along one of two long uteri instead of the single uterus found in a human female. Development at birth is similar, with animals also having a poorly developed sense of vision and other senses.{{Fact|date=March 2007}}
{{-}}


Other heart birth defects include ], ], and ].
==See also==

{{commons|Fetus}}
An ] can result in the death of the fetus and where this is rarely not resolved it can lead to its formation into a ].
* ]

* ]
== Fetal pain ==
* ]
{{main|Prenatal perception}}
* ]

* ]
The existence and implications of ] are debated politically and academically. According to the conclusions of a review published in 2005, "Evidence regarding the capacity for fetal pain is limited but indicates that fetal perception of pain is unlikely before the third trimester."<ref name="JAMA">{{cite journal |last1 = Lee |first1 = Susan |title = Fetal Pain A Systematic Multidisciplinary Review of the Evidence |journal = Journal of the American Medical Association |volume = 294 |issue = 8 |date = August 24–31, 2005 |pmid = 16118385 |doi = 10.1001/jama.294.8.947 |pages = 947–54 |last2 = Ralston |first2 = HJ |last3 = Drey |first3 = EA |last4 = Partridge |first4 = JC |last5 = Rosen |first5 = MA |doi-access = }} Two authors of the study published in JAMA did not report their abortion-related activities, which pro-life groups called a conflict of interest; the editor of JAMA responded that JAMA probably would have mentioned those activities if they had been disclosed, but still would have published the study. See Denise Grady, {{webarchive|url=https://web.archive.org/web/20090425131833/http://www.nytimes.com/2005/08/26/health/26pain.html |date=2009-04-25 }}, ''New York Times'' (2005-08-26).</ref><ref name="sskqke"> NBC News</ref> However, developmental ] argue that the establishment of ] connections (at about {{frac|6|1|2}} months) is an essential event with regard to fetal perception of pain.<ref name="Johnson">Johnson, Martin and Everitt, Barry. '''' (Blackwell 2000): "The multidimensionality of pain perception, involving sensory, emotional, and cognitive factors may in itself be the basis of conscious, painful experience, but it will remain difficult to attribute this to a fetus at any particular developmental age." Retrieved 2007-02-21.</ref>{{Page needed|date=May 2022}} Nevertheless, the perception of pain involves sensory, emotional and cognitive factors and it is "impossible to know" when pain is experienced, even if it is known when thalamocortical connections are established.<ref name="Johnson"/> Some authors argue that fetal pain is possible from the second half of ]. Evidence suggests that the perception of pain in the fetus occurs well before late gestation.<ref name="Anand">{{cite web |url=http://www.iasp-pain.org/AM/AMTemplate.cfm?Section=HOME&TEMPLATE=/CM/ContentDisplay.cfm&CONTENTID=15390&SECTION=HOME |title=Fetal pain? |date=June 2006 |website=International Association for the Study of Pain |archive-url=https://web.archive.org/web/20130701122731/http://www.iasp-pain.org/AM/AMTemplate.cfm?Section=HOME&TEMPLATE=%2FCM%2FContentDisplay.cfm&CONTENTID=15390&SECTION=HOME |archive-date=2013-07-01}}</ref>
* ]

Whether a fetus has the ability to feel pain and ] is part of the ].<ref>{{cite web |quote="The neural pathways are present for pain to be experienced quite early by unborn babies," explains Steven Calvin, M.D., perinatologist, chair of the Program in Human Rights Medicine, University of Minnesota, where he teaches obstetrics." |url=http://www.mccl.org/unborn-babies-can-feel-pain.html |title=Unborn babies can feel pain |archive-url=https://web.archive.org/web/20160719130222/http://www.mccl.org/unborn-babies-can-feel-pain.html|archive-date=2016-07-19 |website= Minnesota Citizens Concerned for Life }}</ref><ref>{{cite journal |url=http://www.asahq.org/Newsletters/2001/10_01/white.htm |title=Are We Overlooking Fetal Pain and Suffering During Abortion? |journal=American Society of Anesthesiologists Newsletter |date=October 2001 |access-date=2007-03-10 |volume=65 |first1=R. Frank |last1=White |url-status=dead |archive-url=https://web.archive.org/web/20080125063355/http://www.asahq.org/Newsletters/2001/10_01/white.htm |archive-date= Jan 25, 2008 }}</ref><ref>David, Barry and Goldberg, Barth. " {{webarchive|url=https://web.archive.org/web/20070928035813/http://www.illinoisbar.org/IBJ/dec02lj/dectoc.htm |date=2007-09-28 }}", ''Illinois Bar Journal'' (December 2002). Retrieved 2007-03-10.</ref> In the United States, for example, ] have proposed legislation that would require providers of abortions to inform pregnant women that their fetuses may feel pain during the procedure and that would require each person to accept or decline ] for the fetus.<ref>Weisman, Jonathan. " {{webarchive|url=https://web.archive.org/web/20081028163732/http://www.washingtonpost.com/wp-dyn/content/article/2006/12/04/AR2006120401089.html |date=2008-10-28 }}", ''Washington Post'' 2006-12-05. Retrieved 2007-02-06.</ref>

== Legal and social issues ==
{{expand section|date=June 2022}}
] and/or tolerated in most countries, although with gestational time limits that normally prohibit ]s.<ref>Anika Rahman, Laura Katzive and Stanley K. Henshaw. " {{webarchive|url=https://web.archive.org/web/20160303230608/http://www.guttmacher.org/pubs/journals/2405698.pdf |date=2016-03-03 }}", ''International Family Planning Perspectives'' Volume 24, Number 2 (June 1998).</ref>

==Other animals==
{{further|Evolution of mammals}}
]

A fetus is a stage in the ] of ] organisms. This stage lies between ] and birth.<ref name=":0" /> Many vertebrates have fetal stages, ranging from most mammals to many fish. In addition, some invertebrates bear live young, including some species of ]<ref name=ony>{{Cite journal |year = 1995 |journal = Journal of Morphology |volume = 224 |pages = 179–198 |doi = 10.1002/jmor.1052240207 |pmid = 29865325 |title = Developing embryo and cyclic changes in the uterus of Peripatus (Macroperipatus) acacioi (Onychophora, Peripatidae) |author = Campiglia, Sylvia S. |last2 = Walker |first2 = Muriel H. |issue = 2 |s2cid = 46928727 }}</ref> and many ]s.

The fetuses of most mammals are situated similarly to the human fetus within their mothers.<ref>ZFIN, {{webarchive|url=https://web.archive.org/web/20070714230655/http://zfin.org/zf_info/zfbook/stages/phar.html |date=2007-07-14 }}. Modified from: Kimmel et al., 1995. Developmental Dynamics 203:253–310. Downloaded 5 March 2007.</ref> However, the anatomy of the area surrounding a fetus is different in ] compared to humans: each fetus of a litter-bearing animal is surrounded by placental ] and is lodged along one of two long uteri instead of the single uterus found in a human female.

Development at birth varies considerably among animals, and even among mammals. ] species are relatively helpless at birth and require considerable parental care and protection. In contrast, ] animals are born with open eyes, have hair or down, have large brains, and are immediately mobile and somewhat able to flee from, or defend themselves against, ]. ]s are precocial at birth, with the exception of humans.<ref>Lewin, Roger. {{Webarchive|url=https://web.archive.org/web/20230407212143/https://books.google.com/books?id=SopsLRo1QyUC&pg=PA78 |date=2023-04-07 }}, page 78 (Blackwell 2004).</ref>

The duration of gestation in ] varies from 18 days in ] to 23 months in ]s.<ref name="Sumich">Sumich, James and Dudley, Gordon. , page 320 (Jones & Bartlett 2008).</ref> Generally speaking, fetuses of larger land mammals require longer gestation periods.<ref name="Sumich" />

]]]

The benefits of a fetal stage means that young are more developed when they are born. Therefore, they may need less parental care and may be better able to fend for themselves. However, carrying fetuses exerts costs on the mother, who must take on extra food to fuel the growth of her offspring, and whose mobility and comfort may be affected (especially toward the end of the fetal stage).

In some instances, the presence of a fetal stage may allow organisms to time the birth of their offspring to a favorable season.<ref name=ony/>
{{Clear}}

== See also ==
{|
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* ] * ]
* ] * ]
* ]
* ]
|
* ]
* ]
* ]
* ]
|}


==References== == References ==
{{reflist|colwidth=35em}} {{reflist|30em}}


==External links== == External links ==
{{Commons}}
*
* at the Endowment for Human Development website, featuring numerous motion pictures of human fetal movement.
* (National Geographic video).
* : MedlinePlus Medical Encyclopedia


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Latest revision as of 03:41, 4 December 2024

Prenatal organism between the embryonic state and birth This article is about the stage of prenatal development. For other uses, see Fetus (disambiguation). "Foetus" redirects here. For the band, see Foetus (band). For the film, see Foetus (film).

Part of a series on
Human growth
and development
Stages
Biological milestones
Development and psychology
Developmental stage theories

A fetus or foetus (/ˈfiːtəs/; pl.: fetuses, foetuses, rarely feti or foeti) is the unborn mammalian offspring that develops from an embryo. Following the embryonic stage, the fetal stage of development takes place. Prenatal development is a continuum, with no clear defining feature distinguishing an embryo from a fetus. However, in general a fetus is characterized by the presence of all the major body organs, though they will not yet be fully developed and functional, and some may not yet be situated in their final anatomical location.

In human prenatal development, fetal development begins from the ninth week after fertilization (which is the eleventh week of gestational age) and continues until the birth of a newborn.

Etymology

The word fetus (plural fetuses or rarely feti) comes from Latin fētus 'offspring, bringing forth, hatching of young'. The Latin plural fetūs is not used in English; occasionally the plural feti is used in English by analogy with second-declension Latin nouns ending in -us.

The predominant British, Irish, and Commonwealth spelling is foetus, except in medical usage, where fetus is preferred. The -oe- spelling is first attested in 1594 and arose in Late Latin by analogy with classical Latin words like amoenus.

Development in humans

Further information: Prenatal development

Weeks 9 to 16 (2 to 3.6 months)

A human fetus, attached to placenta, at three months gestational age

In humans, the fetal stage starts nine weeks after fertilization. At this time the fetus is typically about 30 millimetres (1+1⁄4 in) in length from crown to rump, and weighs about 8 grams. The head makes up nearly half of the size of the fetus. Breathing-like movements of the fetus are necessary for the stimulation of lung development, rather than for obtaining oxygen. The heart, hands, feet, brain, and other organs are present, but are only at the beginning of development and have minimal operation. Uncontrolled movements and twitches occur as muscles, the brain, and pathways begin to develop.

Weeks 17 to 25 (3.6 to 6.6 months)

A woman pregnant for the first time (nulliparous) typically feels fetal movements at about 21 weeks, whereas a woman who has given birth before will typically feel movements by 20 weeks. By the end of the fifth month, the fetus is about 20 cm (8 in) long.

Weeks 26 to 38 (6.6 to 8.6 months)

The amount of body fat rapidly increases. Lungs are not fully mature. Neural connections between the sensory cortex and thalamus develop as early as 24 weeks of gestational age, but the first evidence of their function does not occur until around 30 weeks. Bones are fully developed but are still soft and pliable. Iron, calcium, and phosphorus become more abundant. Fingernails reach the end of the fingertips. The lanugo, or fine hair, begins to disappear until it is gone except on the upper arms and shoulders. Small breast buds are present in both sexes. Head hair becomes coarse and thicker. Birth is imminent and occurs around the 38th week after fertilization. The fetus is considered full-term between weeks 37 and 40 when it is sufficiently developed for life outside the uterus. It may be 48 to 53 cm (19 to 21 in) in length when born. Control of movement is limited at birth, and purposeful voluntary movements continue to develop until puberty.

Variation in growth

Further information: Birth weight and Environmental toxicants and fetal development

There is much variation in the growth of the human fetus. When the fetal size is less than expected, the condition is known as intrauterine growth restriction also called fetal growth restriction; factors affecting fetal growth can be maternal, placental, or fetal.

  • Placental factors include size, microstructure (densities and architecture), umbilical blood flow, transporters and binding proteins, nutrient utilization, and nutrient production.
  • Fetal factors include the fetal genome, nutrient production, and hormone output. Also, female fetuses tend to weigh less than males, at full term.

Fetal growth is often classified as follows: small for gestational age (SGA), appropriate for gestational age (AGA), and large for gestational age (LGA). SGA can result in low birth weight, although premature birth can also result in low birth weight. Low birth weight increases the risk for perinatal mortality (death shortly after birth), asphyxia, hypothermia, polycythemia, hypocalcemia, immune dysfunction, neurologic abnormalities, and other long-term health problems. SGA may be associated with growth delay, or it may instead be associated with absolute stunting of growth.

Viability

Main article: Fetal viability Stages in prenatal development, showing viability and point of 50% chance of survival at bottom. Weeks and months numbered by gestation.

Fetal viability refers to a point in fetal development at which the fetus may survive outside the womb. The lower limit of viability is approximately 5+3⁄4 months gestational age and is usually later.

There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable. According to data from 2003 to 2005, survival rates are 20–35% for babies born at 23 weeks of gestation (5+3⁄4 months); 50–70% at 24–25 weeks (6 – 6+1⁄4 months); and >90% at 26–27 weeks (6+1⁄2 – 6+3⁄4 months) and over. It is rare for a baby weighing less than 500 g (1 lb 2 oz) to survive.

When such premature babies are born, the main causes of mortality are that neither the respiratory system nor the central nervous system are completely differentiated. If given expert postnatal care, some preterm babies weighing less than 500 g (1 lb 2 oz) may survive, and are referred to as extremely low birth weight or immature infants.

Preterm birth is the most common cause of infant mortality, causing almost 30 percent of neonatal deaths. At an occurrence rate of 5% to 18% of all deliveries, it is also more common than postmature birth, which occurs in 3% to 12% of pregnancies.

Circulatory system

Main article: Fetal circulation

Before birth

Diagram of the human fetal circulatory system

The heart and blood vessels of the circulatory system form relatively early during embryonic development, but continue to grow and develop in complexity in the growing fetus. A functional circulatory system is a biological necessity since mammalian tissues can not grow more than a few cell layers thick without an active blood supply. The prenatal circulation of blood is different from postnatal circulation, mainly because the lungs are not in use. The fetus obtains oxygen and nutrients from the mother through the placenta and the umbilical cord.

Blood from the placenta is carried to the fetus by the umbilical vein. About half of this enters the fetal ductus venosus and is carried to the inferior vena cava, while the other half enters the liver proper from the inferior border of the liver. The branch of the umbilical vein that supplies the right lobe of the liver first joins with the portal vein. The blood then moves to the right atrium of the heart. In the fetus, there is an opening between the right and left atrium (the foramen ovale), and most of the blood flows from the right into the left atrium, thus bypassing pulmonary circulation. The majority of blood flow is into the left ventricle from where it is pumped through the aorta into the body. Some of the blood moves from the aorta through the internal iliac arteries to the umbilical arteries and re-enters the placenta, where carbon dioxide and other waste products from the fetus are taken up and enter the mother's circulation.

Some of the blood from the right atrium does not enter the left atrium, but enters the right ventricle and is pumped into the pulmonary artery. In the fetus, there is a special connection between the pulmonary artery and the aorta, called the ductus arteriosus, which directs most of this blood away from the lungs (which are not being used for respiration at this point as the fetus is suspended in amniotic fluid).

Postnatal development

Main article: Adaptation to extrauterine life

With the first breath after birth, the system changes suddenly. Pulmonary resistance is reduced dramatically, prompting more blood to move into the pulmonary arteries from the right atrium and ventricle of the heart and less to flow through the foramen ovale into the left atrium. The blood from the lungs travels through the pulmonary veins to the left atrium, producing an increase in pressure that pushes the septum primum against the septum secundum, closing the foramen ovale and completing the separation of the newborn's circulatory system into the standard left and right sides. Thereafter, the foramen ovale is known as the fossa ovalis.

The ductus arteriosus normally closes within one or two days of birth, leaving the ligamentum arteriosum, while the umbilical vein and ductus venosus usually closes within two to five days after birth, leaving, respectively, the liver's ligamentum teres and ligamentum venosus.

Immune system

The placenta functions as a maternal-fetal barrier against the transmission of microbes. When this is insufficient, mother-to-child transmission of infectious diseases can occur.

Maternal IgG antibodies cross the placenta, giving the fetus passive immunity against those diseases for which the mother has antibodies. This transfer of antibodies in humans begins as early as the fifth month (gestational age) and certainly by the sixth month.

Developmental problems

Further information: Environmental toxicants and fetal development and Birth defect

A developing fetus is highly susceptible to anomalies in its growth and metabolism, increasing the risk of birth defects. One area of concern is the lifestyle choices made during pregnancy. Diet is especially important in the early stages of development. Studies show that supplementation of the person's diet with folic acid reduces the risk of spina bifida and other neural tube defects. Another dietary concern is whether breakfast is eaten. Skipping breakfast could lead to extended periods of lower than normal nutrients in the maternal blood, leading to a higher risk of prematurity, or birth defects.

Alcohol consumption may increase the risk of the development of fetal alcohol syndrome, a condition leading to intellectual disability in some infants. Smoking during pregnancy may also lead to miscarriages and low birth weight (2,500 grams (5 pounds 8 ounces). Low birth weight is a concern for medical providers due to the tendency of these infants, described as "premature by weight", to have a higher risk of secondary medical problems.

X-rays are known to have possible adverse effects on the development of the fetus, and the risks need to be weighed against the benefits.

Congenital disorders are acquired before birth. Infants with certain congenital heart defects can survive only as long as the ductus remains open: in such cases the closure of the ductus can be delayed by the administration of prostaglandins to permit sufficient time for the surgical correction of the anomalies. Conversely, in cases of patent ductus arteriosus, where the ductus does not properly close, drugs that inhibit prostaglandin synthesis can be used to encourage its closure, so that surgery can be avoided.

Other heart birth defects include ventricular septal defect, pulmonary atresia, and tetralogy of Fallot.

An abdominal pregnancy can result in the death of the fetus and where this is rarely not resolved it can lead to its formation into a lithopedion.

Fetal pain

Main article: Prenatal perception

The existence and implications of fetal pain are debated politically and academically. According to the conclusions of a review published in 2005, "Evidence regarding the capacity for fetal pain is limited but indicates that fetal perception of pain is unlikely before the third trimester." However, developmental neurobiologists argue that the establishment of thalamocortical connections (at about 6+1⁄2 months) is an essential event with regard to fetal perception of pain. Nevertheless, the perception of pain involves sensory, emotional and cognitive factors and it is "impossible to know" when pain is experienced, even if it is known when thalamocortical connections are established. Some authors argue that fetal pain is possible from the second half of pregnancy. Evidence suggests that the perception of pain in the fetus occurs well before late gestation.

Whether a fetus has the ability to feel pain and suffering is part of the abortion debate. In the United States, for example, anti-abortion advocates have proposed legislation that would require providers of abortions to inform pregnant women that their fetuses may feel pain during the procedure and that would require each person to accept or decline anesthesia for the fetus.

Legal and social issues

This section needs expansion. You can help by adding to it. (June 2022)

Abortion of a human pregnancy is legal and/or tolerated in most countries, although with gestational time limits that normally prohibit late-term abortions.

Other animals

Further information: Evolution of mammals
Fourteen phases of elephant development before birth

A fetus is a stage in the prenatal development of viviparous organisms. This stage lies between embryogenesis and birth. Many vertebrates have fetal stages, ranging from most mammals to many fish. In addition, some invertebrates bear live young, including some species of onychophora and many arthropods.

The fetuses of most mammals are situated similarly to the human fetus within their mothers. However, the anatomy of the area surrounding a fetus is different in litter-bearing animals compared to humans: each fetus of a litter-bearing animal is surrounded by placental tissue and is lodged along one of two long uteri instead of the single uterus found in a human female.

Development at birth varies considerably among animals, and even among mammals. Altricial species are relatively helpless at birth and require considerable parental care and protection. In contrast, precocial animals are born with open eyes, have hair or down, have large brains, and are immediately mobile and somewhat able to flee from, or defend themselves against, predators. Primates are precocial at birth, with the exception of humans.

The duration of gestation in placental mammals varies from 18 days in jumping mice to 23 months in elephants. Generally speaking, fetuses of larger land mammals require longer gestation periods.

Fetal stage of a porpoise

The benefits of a fetal stage means that young are more developed when they are born. Therefore, they may need less parental care and may be better able to fend for themselves. However, carrying fetuses exerts costs on the mother, who must take on extra food to fuel the growth of her offspring, and whose mobility and comfort may be affected (especially toward the end of the fetal stage).

In some instances, the presence of a fetal stage may allow organisms to time the birth of their offspring to a favorable season.

See also

References

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