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A fetus (or foetus, or fœtus) is a developing mammal or other viviparous vertebrate, after the embryonic stage and before birth. The plural is fetuses (foetuses, fœtuses) or, very rarely, foeti.

In humans, a fetus develops from the end of the eighth week after fertilisation, when the major structures and organ systems have formed, until birth. The word fetus is a Latin word having Indo-European roots.

Etymology and spelling variations

The word "fetus" is derived from the Latin fetus, meaning "offspring", "bringing forth", or "hatching of young". It has Indo-European roots related to sucking or suckling.

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 Oxford English Dictionary, which describes "fetus" as the etymologically preferable spelling. The English variation "foetus" may have originated with an error by Saint Isidore of Seville, in AD 620. The medical community prefers the spelling fetus, but the pseudo-Greek spelling foetus persists in general use, especially in the UK, Australia, and New Zealand.

Human fetus

The fetal stage begins eight weeks after fertilisation. 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 miscarriage, feticide committed by a third party, or abortion chosen by the pregnant woman.

Development

The following timeline describes some of the specific changes in fetal anatomy and physiology by fertilization age (i.e. the time elapsed since fertilization). However, it should be noted that obstetricians often use "gestational age" which, by convention, is measured from 2 weeks earlier than fertilization. For purposes of this article, age is measured from fertilization.

• 8 Weeks (condition at start of fetal stage). The risk of miscarriage decreases sharply at the beginning of the fetal stage. 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. When the fetal stage commences, a human fetus is typically about 30 mm (1.2 inches) in length, and the heart is beating. The fetus bends the head, and also makes general movements and startles that involve the whole body. Brain stem activity has been detected as early as 54 days after conception. Some fingerprint formation can be seen from the beginning of the fetal stage.

• 8 to 15 Weeks. The fetus continues to move in distinct motor patterns, picking up new patters such as localized movement of the arms and legs, hiccups, breathing-like movements, and stretches and yawns . The breathing-like movement of the fetus is necessary for stimulation of lung development, rather than for obtaining oxygen or intake of amniotic fluid. 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. 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. Tooth buds appear, the limbs are long and thin, and red blood cells are produced in the liver. A fine hair called lanugo develops on the head. The gastrointestinal tract, still forming, starts to collect sloughed skin and lanugo, as well as hepatic products, forming meconium (stool). Fetal skin is almost transparent. More muscle tissue and bones have developed, and the bones become harder. The first measurable signs of EEG activity occur in the 12th week. By the end of this stage, the fetus has reached about 15 cm (6 inches).

• 16 to 25 Weeks. The lanugo covers the entire body. Eyebrows, eyelashes, fingernails, and toenails appear. The fetus has increased muscle development. Alveoli (air sacs) are forming in lungs. The nervous system develops enough to control some body functions. The cochlea are now developed, though the myelin 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 quickening, 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.

• 26 to 38 Weeks. The amount of body fat rapidly increases. Lungs are not fully mature. Thalamic brain connections, which mediate sensory input, form. Bones are fully developed, but are still soft and pliable. Iron, calcium, and phosphorus become more abundant. Continuous EEG readings have been observed by the 30th week. Fingernails reach the end of the fingertips. The lanugo begins to disappear, until it is gone except on the upper arms and shoulders. Small breast buds are present on both sexes. Head hair becomes coarse and thicker. Birth is imminent and occurs around the 40th gestational week. By the end of the fifth month, the fetus is about 20 cm (8 inches). The fetus is considered full-term by week 35, which means that the baby is considered sufficiently developed for life outside the womb. It may be 48 to 53 cm (19 to 21 inches) in length, when born.

Variation in growth

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, placental, or fetal.

Maternal factors include maternal weight, body mass index, nutritional state, emotional stress, toxin exposure (including tobacco, alcohol, heroin, and other drugs which can also harm the fetus in other ways), and uterine blood flow. A woman's primiparity also may affect fetal weight (firstborns tend to weigh less).

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

Fetal factors include the fetus genome, nutrient production, and hormone output. Also, females tend to weigh less than males.

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 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

Just under six months is the earliest point at which medical science has been able to achieve viability, which usually occurs later. According to The Developing Human:

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.

During the past several decades, expert postnatal care has improved with advances in medical science, and therefore the point of viability has moved toward conception.

Fetal pain

The subject of fetal pain and suffering is controversial. There is an "emerging consensus among developmental neurobiologists that the establishment of thalamocortical connections" (at about 26 weeks) is a critical event with regard to fetal pain. However, because pain perception involves "sensory, emotional, and cognitive factors," it is "impossible to know" when painful experiences may become possible, even if it is known when thalamocortical connections are established.

The ability of a fetus to feel pain is often part of the abortion debate. Laws have been proposed by social conservatives to require that fetuses be anesthetized before an abortion in order to reduce the possibility of fetal pain.

Circulatory system

The circulatory system of a human fetus works differently from that of born humans, mainly because the lungs are not in use: the fetus obtains oxygen and nutrients from the woman through the placenta and the umbilical cord.

Blood from the placenta is carried by the umbilical vein. About half of this enters the 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 woman'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 aren't being used for respiration at this point as the fetus is suspended in amniotic fluid).

Postnatal development

With the first breath after birth, the system changes suddenly. The pulmonary resistance is dramatically reduced ("pulmo" is from the Latin for "lung"). More blood moves from the right atrium to the right ventricle and into the pulmonary arteries, and less flows through the foramen ovale 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.

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 ligamentum teres and the ligamentum venosus of the liver respectively.

Differences from the adult circulatory system

Remnants of the fetal circulation can be found in adults:

In addition to differences in circulation, the developing fetus also employs a different type of oxygen transport molecule than adults (adults use adult hemoglobin). Fetal hemoglobin 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.

Developmental problems

Congenital anomolies are anomolies 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 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.

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 during the first trimester of development. Studies show that supplementation of the mother's diet with folic acid reduces the risk of spina bifida and other neural tube 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 prematurity, or other birth defects in the fetus. During this time alcohol consumption may increase the risk of the development of Fetal Alcohol Spectrum Disorder, a condition leading to mental retardation in some infants. Smoking during pregnancy may also lead to a low birth weight infant, with a weight of <2500 grams, or 5.5 lbs. 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.

Legal issues

Especially since the 1970s, there has been continuing debate over the "personhood" of the human fetus. Although abortion of a fetus before viability is generally legal in the United States following the case of Roe v. Wade, the third-party-killing of a fetus can be punishable as feticide or homicide even before viability, depending on jurisdiction.

Non-human fetuses

This section needs expansion. You can help by adding to it.

The fetus of most mammals develops similarly to the Homo sapiens fetus. After the first stages of development, the human embryo reaches a stage very similar to all other vertebrates. The anatomy of the area surrounding a fetus is different in litter-bearing animals compared to humans: each fetus 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 is similar, with animals also having a poorly developed sense of vision and other senses.

References

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6. Greenfield, Marjorie. “Dr. Spock.com". Retrieved 2007-01-20.
7. ^ Prechtl, Heinz. "Prenatal and Early Postnatal Development of Human Motor Behavior" in Handbook of brain and behaviour in human development, Kalverboer and Gramsbergen eds., pp. 415-418 (2001 Kluwer Academic Publishers). Retrieved 2007-03-04.
8. ^ Singer, Peter. Rethinking life & death: the collapse of our traditional ethics, page 104 (St. Martins Press 1996). Retrieved 2007-03-04.
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10. Institute of Medicine of the National Academies, Preterm Birth: Causes, Consequences, and Prevention (2006). Retrieved 2007-03-04.
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18. Roe v. Wade, 410 U.S. 113 (1973) ("viability is usually placed at about seven months (28 weeks) but may occur earlier, even at 24 weeks.") Retrieved 2007-03-04.
19. ^ Johnson, Martin and Everitt, Barry. Essential reproduction (Blackwell 2000), pages 215-216: "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.
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External links

• High Resolution Nissl-stained Rat Fetus

See also

• Embryo
• Prenatal development
• Pregnancy
• Child
• Superfetation
• Neural development
• Fetoscopy
• Fetal position
• Abortion

• Developmental biology