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The term trimester redirects here. For the term trimester used in academic settings, see Academic term

Pregnancy is the carrying of one or more embryos or fetuses by female mammals, including humans, inside their bodies. In a pregnancy, there can be multiple gestations (for example, in the case of twins, or triplets). Human pregnancy is the most studied of all mammalian pregnancies.

Human pregnancy lasts approximately 38 weeks from conception to childbirth. The medical term for a pregnant woman is genetalian, just as the medical term for the baby is embryo (early weeks) and then fetus (until birth). A woman who is pregnant for the first time is known as a primigravida or gravida 1: a woman who has never been pregnant is known as a gravida 0; similarly, the terms para 0, para 1 and so on are used for the number of times a woman has given birth.

In many societies' medical and legal definitions, human pregnancy is somewhat arbitrarily divided into three trimester periods, as a means to simplify reference to the different events and symptoms during the pregnancy. However, in terms of prenatal development, human pregnancy is instead broken up differently (i.e. into a pre-implantation period of two weeks from conception, an embryonic period of six more weeks, and a fetal period until birth). Nevertheless, the trimester framework is useful in describing pregnancy from the pregnant woman's point of view. The first trimester period carries the highest risk of miscarriage (natural death of embryo or fetus). During the second trimester the development of the fetus can start to be monitored and diagnosed. The third trimester marks the beginning of viability, or the ability of the fetus to survive, with or without medical help, outside of the mother's womb.

See also Pregnancy terms and definitions

Determining the beginning of pregnancy and predicting date of birth

Before pregnancy begins, a female oocyte (egg) must join, by spermatozoon in a process referred to in medicine as "fertilisation", or commonly (though perhaps inaccurately) as "conception." This occurs usually through the act of sexual intercourse, in which a man ejaculates inside a woman, thus releasing his sperm; however, the advent of artificial insemination has made it possible for women to become pregnant if prexisting medical conditions from either the woman or the man make fertilisation through sexual intercourse difficult, or if a woman chooses to become pregnant without a male partner, for any number of reasons. Though pregnancy begins at implantation, it is often convenient to date from the first day of a woman's last menstrual period. This is used to calculate the Estimated Date of Delivery (EDD).

Traditionally (according to Naegele's rule, which is used to calculate the estimated date of delivery, or EDD), a human pregnancy is considered to last approximately 40 weeks (280 days) from the last menstrual period (LMP), or 37 weeks (259 days) from the date of fertilization. However, a pregnancy is considered to have reached term between 37 and 43 weeks from the beginning of the last menstruation. Babies born before the 37 week mark are considered premature, while babies born after the 43 week mark are considered post-mature.

According to the Merck Medical Manual], the norm for human pregnancy is approximately 266 days from the date of fertilization. This is 38 weeks, or 9 lunar months. On the more familiar Gregorian calendar, it is a little less than nine months: about 8 months and 22.5 days. Counting from the beginning of the woman's last menstrual cycle, the norm is 40 weeks (the basis for Naegele's rule).

Though these are the averages, the actual length of pregnancy depends on various factors. For example, the first pregnancy tends to last longer than subsequent pregnancies. Fewer than 10% of births occur on the due date; 50% of births are within a week of the due date, and almost 90% within two weeks.

The beginning of pregnancy may be detected in a number of ways, including various pregnancy tests which detect hormones generated by the newly-formed placenta. Clinical blood and urine tests can detect pregnancy soon after implantation, which is as early as 6-8 days after fertilization. Home pregnancy tests are personal urine tests, which normally cannot detect a pregnancy until at least 12-15 days after fertilization. Both clinical and home tests can only detect the state of pregnancy, and cannot detect its age.

In the post-implantation phase, the blastocyst secretes a hormone named human chorionic gonadotropin which in turn, stimulates the corpus luteum in the woman's ovary to continue producing progesterone. This acts to maintain the lining of the uterus so that the embryo will continue to be nourished. The glands in the lining of the uterus will swell in response to the blastocyst, and capillaries will be stimulated to grow in that region. This allows the blastocyst to receive vital nutrients from the woman. Pregnancy tests detect the presence of human chorionic gonadotropin.

An early ultrasound can determine the age of the pregnancy fairly accurately. In practice, doctors typically express the age of a pregnancy (i.e. an "age" for an embryo) in terms of "menstrual date" based on the first day of a woman's last menstrual period, as the woman reports it. Unless a woman's recent sexual activity has been limited, the exact date of fertilization is unknown. Absent symptoms such as morning sickness, often the only visible sign of a pregnancy is an interruption of her normal monthly menstruation cycle, (i.e. a "late period"). Hence, the "menstrual date" is simply a common educated estimate for the age of a fetus, which is an average of two weeks later than the first day of the woman's last menstrual period. (The margin of error is 0 to 30 days after last menstruation, hence a 14 day average.) The term "conception date" may sometimes be used when that date is more certain, though even medical professionals can be imprecise with their use of the two distinct terms. The due date can be calculated by using Naegele's rule.

An ultrasound of a developing fetus.

There are likewise finer distinctions between the concepts of fertilization and the actual state of pregnancy, which starts with implantation. In a normal pregnancy, the fertilization of the egg usually will have occurred in the Fallopian tubes or in the uterus. (Often, an egg may become fertilized yet fail to become implanted in the uterus.) If the pregnancy is the result of in-vitro fertilization, the fertilization will have occurred in a Petri dish, after which pregnancy begins when one or more zygotes implant after being transferred by a physician into the woman's uterus.

In the context of political debates regarding a proper definition of life, the terminology of pregnancy can be confusing. Because precise assessment of a pregnancy as being at the "embryo" or "fetus" stage is usually undeterminable, the terms (though more clinically precise) are less commonly used than terms like "baby" or "child." The medically and politically neutral term which remains is simply "pregnancy," though this can be problematic as it only refers indirectly to the embryo or fetus. In the context of personal treatment, bedside manner generally dictates that doctors make sparse use of clinical language like "fetus" and "embryo," and instead simply refer to the developing child as a "baby", though this is not medically accurate.

Detection

The early stages of pregnancy is often discovered by using a pregnancy test. In the post-implantation phase the blastocyst secretes a hormone named human chorionic gonadotropin which in turn, stimulates the corpus luteum in the woman's ovary to continue producing progesterone. This acts to maintain the lining of the uterus so that the embryo will continue to be nourished. The glands in the lining of the uterus will swell in response to the blastocyst, and capillaries will be stimulated to grow in that region. This allows the blastocyst to receive vital nutrients from the woman. Pregnancy tests detect the presence of human chorionic gonadotropin.

Timeline of a typical pregnancy

Pregnancy is typically broken into three periods, or trimesters, each of about three months. However, in terms of biology and prenatal development, human pregnancy is instead broken up differently (i.e. into a pre-implantation period of two weeks from conception, an embryonic period of six more weeks, and a fetal period until birth). Nevertheless, the trimester framework is useful in describing the changes that take place over time, especially from the pregnant woman's point of view.

First trimester

In medicine, pregnancy is defined as the period from conception to birth. However, it is not uncommon for doctors to say that pregnancy begins after conception, when the developing embryo becomes implanted into the endometrial lining of a woman's uterus. Alternatively, obstetricians often date weeks of pregnancy from before conception, i.e. from the start of the last menstrual period which means that conception would typically occur at the end of the 2nd week of pregnancy.

In some cases where complications may have arisen, the fertilized egg might implant itself in the fallopian tubes or the cervix, causing an ectopic pregnancy. Most pregnant women do not have any specific signs or symptoms of implantation, although it is not uncommon to experience light bleeding at implantation. Some women will also experience cramping during their first trimester. This is usually of no concern unless there is spotting or bleeding as well. The outer layers of the embryo grow and form a placenta, for the purpose of receiving essential nutrients through the uterine wall, or endometrium. The umbilical cord in a newborn child consists of the remnants of the connection to the placenta. The developing embryo undergoes tremendous growth and changes during the process of embryonic and fetal development.

Morning sickness afflicts about seventy percent of all pregnant women, typically only in the first trimester. Most miscarriages occur during this period.

Second trimester

Months 4 through 6 of the pregnancy are called the second trimester. Most women feel more energized in this period, and begin to put on weight as the symptoms of morning sickness subside and eventually fade away. The first movement of the fetus, often referred to as "quickening", can be felt, as it begins to form into a recognizable shape. This typically happens by the fourth month. The placenta is now fully functioning and the fetus is making insulin and urinating. The teeth are now formed inside the fetus' gums and the reproductive organs can be recognized, and can distinguish the fetus as male or female.

Third trimester

Final weight gain takes place, and the fetus begins to move regularly. The mother's belly button will sometimes "pop" out due to her growing belly. This period of her pregnancy can be uncomfortable, causing symptoms like weak bladder control and back-ache. Movement of the baby becomes stronger and more frequent and the fetus prepares for viability outside the womb through improved brain, eye, and muscle function.

It is during this time that a baby born prematurely may survive. The use of modern medical intensive care technology has greatly increased the probability of premature babies living, and has pushed back the boundary of viability to much earlier dates than would be possible without assistance. In spite of these developments, premature birth remains a major threat to the fetus, and may result in ill-health in later life, even if the baby survives.

Food and nutrition during pregnancy

It is even more important than usual for an expectant mother to eat a healthy diet. Except if she has specific health problem (e.g., diabetes mellitus or edema) most usual nutritional advice can be kept: balancing carbohydrates, fat, and proteins; and eating a variety of foods, including dairy products and several fruits and vegetables, daily. Some specific advice can be given however:

Folic acid (also called folate or Vitamin B9) is strongly needed at the start of pregnancy, and even before conception. Folic acid is needed for the closing of neural tube. It thus prevents spina bifida, a very serious birth defect. Folates (from folia, leaf) are abundant in spinach (fresh, frozen or canned), and are also found in green vegetables, salads, melon, and hummus. In the United States, most wheat products (flour, noodles) are supplemented with folic acid.

Calcium and iron are particularly needed by the rapidly growing fetus. Pregnant women should thus eat enough dairy products (for calcium) and red meat (for iron) if they are not vegetarian. Women are often prescribed iron pills, since many young women get slight anemia (lack of iron leading to few red blood cells). Calcium is effective only if women have enough vitamin D. The best mean to get vitamin D is to sunbathe each day for 10-15 min. Salmon and fatty fishes are also a good vitamin D source.

Fluoride helps to build strong teeth by changing the nature of calcium crystals: if water or salt does not contain fluoride, it is wise to take fluoride mini-pills at the end of pregnancy and during breast-feeding (but high doses are toxic!). In many American cities, drinking water is supplemented with fluoride. Some pregnant women suffer edema, and are told not to eat (too much) salt.

Fat from salmon, trout, tuna, herring, sardine, and mackerel contain long-chain omega 3 (n-3) fatty acids that are needed to build neurone membranes. Thus fatty fish intake during pregnancy helps baby's brain and retina development. However, large fish such as tuna and swordfish may contain too much toxic mercury, and one should balance risks with benefits: fish two or three times a week seems to bring enough good fat, but not too much mercury.

Dangerous bacteria or parasites may contaminate foods, particularly listeria and toxoplasma, toxoplasmosis agent. To avoid those two hazards, hygiene rules should be strictly adhered to: carefully wash fruits and raw vegetables; over-cook remainders, meat and processed meat; avoid raw-milk cheeses (listeria); change daily the cat's bedding and then wash hands (toxoplasma); clean the fridge often with diluted chlorine (then rinse). More precise diet and hygiene advices on a website with links to scientific articles and quantitative data: Pregnancy Diet

Medical aspects of pregnancy

Diagnostic criteria are: In a woman who has regular menstrual cycles and is sexually active, a period delayed by a few days or weeks is suggestive of pregnancy; elevated B-hcG to around 100,000 mIU/mL by 10 weeks of gestation.

Prenatal medical care is of recognized value throughout the developed world. Various Vitamins or supplements are recognized as beneficial during pregnancy. Prenatal multivitamins as well as folic acid as well as the choline available from lecithin have either government approval or published studies supporting their use. Folic acid reduces birth defects. Prenatal Choline derivable from lecithin improves the performance of rats on mental tests throughout the rats entire life. Omega 3 fatty acids support the mental and visual development of infants they are also beneficial postpartum.

Birth

Childbirth is the process in which the baby is born. It is considered by many to be the beginning of a person's life, and age is defined relative to this event in most cultures.

A woman is considered to be in labour when she begins experiencing regular uterine contractions, accompanied by changes of her cervix — primarily effacement and dilation. While childbirth is widely experienced as painful, some women do report painless labours, while others find that concentrating on the birth helps to quicken labor and lessen the sensations. Most women are capable of having a normal birth but sometimes complications arise and a woman may need to undergo a caesarean section

During the time immediately after birth, both baby and mother are hormonally cued to bond, the mother through the release of oxytocin a hormone also released during breastfeeding.

Postnatal period

For topics following on from a successful pregnancy and birth, see:

• Breastfeeding
• Child development

Medical disorders in pregnancy

Usually there aren't any medical problems, however, it is possible if a woman pushes her baby out too fast, it will not give the pelvic or birthing canal enough time to stretch; thus leading to ripped internal and external flesh inside the mother and the possibility of a few broken bones for the new born because a child's structure is at a very vulnerable state at that period. Really serious problems are rare and most can be anticipated and treated effectively. However, problems sometimes develop suddenly and unexpectedly. Regular visits to a doctor or midwife during pregnancy make anticipation of problems possible and improve the chances of having a healthy baby.

Approximately 4 million births occur in the United States each year. A significant proportion of these are complicated by one or more medical disorders . Two decades ago, many medical disorders were contra-indications to pregnancy. Advances in obstetrics, neonatology, obstetric anesthesiology, and medicine have increased the expectation that pregnancy will result in an excellent outcome for both mother and fetus despite most of these conditions.

A rare but possibly under-diagnosed disorder in pregnancy is Hyperemesis gravidarum, a condition in which morning sickness is constant and extreme, resulting in dehydration and malnutrition.

Terms and definitions

Technical

• embryo - conceptus between time of fertilization to 10 weeks of gestation
• FASD - Fetal Alcohol Spectrum Disorder, a clinical term for the effects alcohol can have on the developing fetus
• fetus - from 10 weeks of gestation to time of birth
• Ga Pw-x-y-z - a = number of pregnancies, w = number of term births, x = number of preterm births, y = number of miscarriages, z = number of living children; for example, G4P1-2-1-3 means the woman had a total of 4 pregnancies, of which 1 is of term, 2 are preterm, 1 miscarriage, and 3 total living children (1 term + 2 preterm).
• Gestational age - time from last menstrual period (LMP) up to present
• gravidity (G) - number of times a woman has been pregnant
• infant - time of birth to 1 year of age
• parity (P) - number of pregnancies with a birth beyond 20 weeks GA or an infant weighing more than 500 g
• preterm infant - delivered between 24-37 weeks
• previable infant - delivered prior to 24 weeks
• term infant - delivered between 37-42 weeks
• first trimester - up to 14 weeks of gestation
• second trimester - 14 to 28 weeks of gestation
• third trimester - 28th week to delivery
• viability - minimum age for fetus survival, ca. third trimester
• zygote - from fertilization until second cell division
• full term refers to the end of 36 weeks (nine months) from the first day of the mother's last menstrual period — the end of gestation. If a woman gives birth earlier than this, it is classed as a premature birth.

Euphemisms and colloquialisms

There are a number of euphemisms and colloquialisms for pregnancy.

• "With child" is a slightly archaic euphemism.
• "Expecting" is a common euphemism which indicates that a woman is expecting a baby.
• "In a family way" is used as a euphemism in the southern U.S.
• "Drink out of the well" is sometimes used as a metaphor in the southern U.S.
• "On his road" is sometimes used as a metaphor in the southern U.S. to indicate a baby’s imminent birth.
• "Gone" or "along" may be used to represent gestational time (e.g., "She's really far gone."; "She’s six months along.")
• "Having a bun in the oven" is a metaphor for pregnancy.
• "Drop," "pop," "blow," and "burst" describe the state of imminent labor.
• "In a fix" and "preggers" are slang terms used on the east coast of the U.S.
• "Knocked up" is used in Canada and the United States; the act of impregnating a woman is sometimes referred to as "knocking up." This phrase is considered somewhat vulgar.
• "Up the duff" is used in Australia and the UK, particularly in cases in which the pregnancy was unplanned.

Colloquialisms and metaphors for pregnancy exist universally. In France and throughout Europe, phrases translating to "she’s full" are the most common.

Regional customs

In Korea, China, and Vietnam, age is measured starting from conception to acknowledge that the baby exists within the mother’s body before it is born. Therefore, a newborn baby is considered to be one year old.

The ancient Mayan calendar of 276 days possibly originated from the human gestational cycle, or to indicate the world was created as slowly as a baby develops.

See also

• List of pregnancy-related topics
• Doula

References

• Mittendorf R, Williams MA, Berkey CS, Cotter PF. The length of uncomplicated human gestation. Obstet Gynecol 1990;75:929-32. PMID 2342739.
• Medical Disorders During Pregnancy. N Engl J Med 333:1737, 2002.

Notes

1. One example of such preexisting medical conditions is a low male sperm count

External links

• Early pregnancy: Morning sickness, fatigue and other common symptoms, from MayoClinic.com
• Fetal Development Photos and information
• Normal Pregnancy, Labor, And Delivery (Merck Manual)
• The visible embryo
• Presentation of Human Embryo Development
• Pregnancy Week to Week Information about every week of pregnancy
• More4kids Pregnancy Resources Pregnancy and Childbirth Articles
• www.4woman.gov Healthy Pregnancy

• Pregnancy
• Birth control
• Biological reproduction