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This article is about pregnancy in humans. For pregnancy in other mammals, see Pregnancy (mammals). For pregnancy in fish, see Pregnancy in fish.
Classification and external resources
A pregnant woman
ICD-10 Z33
ICD-9 650
DiseasesDB 10545
MedlinePlus 002398
eMedicine article/259724
MeSH D011247

Pregnancy is the common name for gestation in humans. It is the development of one or more offspring, known as an embryo or fetus, in the uterus. A multiple pregnancy involves more than one embryo or fetus in a single pregnancy, such as with twins.

Childbirth usually occurs about 38 weeks after conception. In women who have a menstrual cycle length of four weeks, this is approximately 40 weeks from the start of the last normal menstrual period (LNMP). Health authorities recommend that women not artificially begin delivery with labor induction or caesarean section before 39 weeks as this amount of time is considered "full term" for the child to develop. Human pregnancy is the most studied of all mammalian pregnancies. Conception can be achieved through sexual intercourse or assisted reproductive technology.

An embryo is the developing offspring during the first 8 weeks following conception, and subsequently the term fetus is used until birth.[1][2] In many societies' medical or legal definitions, human pregnancy is somewhat arbitrarily divided into three trimester periods of three months each, as a means to simplify reference to the different stages of prenatal development. The first trimester carries the highest risk of miscarriage (natural death of embryo or fetus). During the second trimester, the development of the fetus can be more easily monitored and diagnosed. The third trimester is marked by further growth of the fetus and the development of fetal fat stores.[3] The point of fetal viability, or the point in time at which fetal life outside of the uterus is possible, usually coincides with the late second or early third trimesters; babies born at this early point in development are at high risk for having medical conditions and dying.[4]






Timeline of pregnancy by gestational age


Fertilization and implantation in humans




Development of embryo and fetus

The initial stages of human embryogenesis



After about 10 weeks of gestational age, the embryo becomes known as a fetus instead. At the beginning of the fetal stage, the risk of miscarriage decreases sharply,[15] When the fetal stage commences, a fetus is typically about 30 mm (1.2 inches) in length, and the heart can be seen beating via ultrasound; the fetus can be seen making various involuntary motions at this stage.[16] During continued fetal development, the early body systems and structures that were established in the embryonic stage continue to develop. Sex organs begin to appear during the third month of gestation. The fetus continues to grow in both weight and length, although the majority of the physical growth occurs in the last weeks of pregnancy.


Maternal changes

Breast changes as seen during pregnancy. The areolae are larger and darker.


The fetus inside a pregnant woman may be viewed as an unusually successful allograft, since it genetically differs from the woman.[22] The main reason for this success is an increased maternal immune tolerance during pregnancy.

Pregnancy is typically broken into three periods, or trimesters, each of about three months.[23][24] Obstetricians define each trimester as lasting for 14 weeks, resulting in a total duration of 42 weeks, although the average duration of pregnancy is actually about 40 weeks.[25] While there are no hard and fast rules, these distinctions are useful in describing the changes that take place over time.

First trimester


Minute ventilation is increased by 40% in the first trimester.[26] The womb will grow to the size of a lemon by eight weeks. Many symptoms and discomforts of pregnancy like nausea and tender breasts appear in the first trimester.[27]

Second trimester




Third trimester





Determining gestational age



The best method of determining gestational age is ultrasound during the first trimester of pregnancy. This is typically accurate within seven days.[30] This means that fewer than 5 percent of births occur on the day of being 40 weeks of gestational age; 50 percent of births are within a week of this duration, and about 80 percent are within 2 weeks.[29] For the estimation of due date, mobile apps essentially always give consistent estimations compared to each other and correct for leap year, while pregnancy wheels made of paper can differ from each other by 7 days and generally do not correct for leap year.[31]




The stage of pregnancy defined as the beginning of legal fetal viability varies around the world. It sometimes incorporates weight as well as gestational age.[33] It ranges from 16 weeks in Norway, to 20 weeks in the US and Australia, 24 weeks in the UK and 26 weeks in Italy and Spain.[33][34][35]

Timing of childbirth

Further information: Preterm birth and Postterm pregnancy

Stages of pregnancy term
stage starts ends
at 37 weeks
Early term[37] 37 weeks 39 weeks
Full term[37] 39 weeks 41 weeks
Late term[37] 41 weeks 42 weeks
Postterm[37] 42 weeks

In the ideal childbirth labor begins on its own when a woman is "at term".[38] Pregnancy is considered at term when gestation has lasted between 37 and 42 weeks.[37] Unless there is a medical reason to do so, planned delivery of a child should not happen until after the completion of 39 weeks of pregnancy.[37]

Events before completion of 37 weeks are considered preterm.[36] Preterm birth is associated with a range of risks and problems and whenever possible should be avoided in favor of giving birth when the pregnancy is at term.[39]

Sometimes if a woman's water breaks or contractions before 39 weeks, birth is unavoidable.[37] A natural beginning to an early term delivery is usually a physiological sign that the time is right for birth and not usually a cause for worry.[citation needed] Intentionally planning to give birth before 39 weeks by Caesarean section or labor induction, even if considered "at term", results in an increased risk of complications and harm to mother and child.[40] This is from factors including underdeveloped lungs of newborns, infection due to underdeveloped immune system, feeding problems due to underdeveloped brain, and jaundice from underdeveloped liver.[41] Some hospitals in the United States have noted a significant increase in neonatal intensive care unit patients when women schedule deliveries for convenience and are taking steps to reduce induction for non-medical reasons.[41]

Babies born between 39 and 41 weeks gestation have better outcomes than babies born either before or after this range.[37] This special time period is called "full term".[37] Whenever possible, waiting for labor to begin on its own in this time period is best for the health of the mother and baby.[38] Because of the likelihood of increased problems including the need for a c-section, between 39–41 weeks inducing labor without a medical indication is discouraged unless the cervix is favorable.[38]

Events after 42 weeks are considered postterm.[37] When a pregnancy exceeds 42 weeks, the risk of complications for both the woman and the fetus increases significantly.[42][43] Therefore, in an otherwise uncomplicated pregnancy, obstetricians usually prefer to induce labour at some stage between 41 and 42 weeks.[44]


Main article: Childbirth

Childbirth is the process whereby an infant is born.



Postnatal period

Main article: Postnatal




Physical signs

Linea nigra in a woman at 22 weeks pregnant

Most pregnant women experience a number of symptoms,[48] which can signify pregnancy. The symptoms can include nausea and vomiting, excessive tiredness and fatigue, cravings for certain foods that are not normally sought out, and frequent urination particularly during the night.

A number of early medical signs are associated with pregnancy.[49][50] These signs typically appear, if at all, within the first few weeks after conception. Although not all of these signs are universally present, nor are all of them diagnostic by themselves, taken together they make a presumptive diagnosis of pregnancy. These signs include the presence of human chorionic gonadotropin (hCG) in the blood and urine, missed menstrual period, implantation bleeding that occurs at implantation of the embryo in the uterus during the third or fourth week after last menstrual period, increased basal body temperature sustained for over 2 weeks after ovulation, Chadwick's sign (darkening of the cervix, vagina, and vulva), Goodell's sign (softening of the vaginal portion of the cervix), Hegar's sign (softening of the uterus isthmus), and pigmentation of linea alba – Linea nigra, (darkening of the skin in a midline of the abdomen, caused by hyperpigmentation resulting from hormonal changes, usually appearing around the middle of pregnancy).[49][50] Breast tenderness is common during the first trimester, and is more common in women who are pregnant at a young age.[51] Shortly after conception, the nipples and areolas begin to darken due to a temporary increase in hormones.[52] This process continues throughout the pregnancy.



Further information: Pregnancy test

Pregnancy detection can be accomplished using one or more various pregnancy tests,[53] which detect hormones generated by the newly formed placenta, serving as biomarkers of pregnancy. Blood and urine tests can detect pregnancy 12 days after implantation.[54] Blood pregnancy tests are more sensitive than urine tests (giving fewer false negatives).[55] Home pregnancy tests are urine tests, and normally detect a pregnancy 12 to 15 days after fertilization. A quantitative blood test can determine approximately the date the embryo was conceived. Testing 48 hours apart can provide useful information regarding how the pregnancy is doing. A single test of progesterone levels can also help determine how likely a fetus will survive in those with a threatened miscarriage (bleeding in early pregnancy).[56]

Ultrasound image of fetus at 14 weeks (profile)


3D ultrasound image of fetus at 20 weeks

Obstetric ultrasonography can detect some congenital diseases at an early stage, estimate the due date as well as detecting multiple pregnancy.[57] The resultant estimated gestational age and due date of the fetus are slightly more accurate than methods based on last menstrual period.[58] In those who are at low risk it is unclear if obstetric ultrasound before 24 weeks makes a significant difference in outcomes.[59]


Attending prenatal care






Several micronutrients are important for the health of the developing fetus, especially in areas of the world where insufficient nutrition is prevalent.[64] In developed areas, such as Western Europe and the United States, certain nutrients such as Vitamin D and calcium, required for bone development, may require supplementation.[65][66][67]


Weight gain

The amount of healthy weight gain during a pregnancy varies.[69] Weight gain is only partly related to the weight of the baby and growing placenta, and includes extra fluid for circulation, and the weight needed to provide nutrition for the growing fetus.[70] Most needed weight gain occurs later in pregnancy.[70]


During pregnancy, insufficient or excessive weight gain can compromise the health of the mother and fetus.[70] The most effective interventions for weight gain in underweight women is not clear.[70] Being or becoming very overweight in pregnancy increases the risk of complications for mother and fetus, including cesarean section, gestational hypertension, pre-eclampsia, macrosomia and shoulder dystocia.[69] It can make losing weight after the pregnancy difficult.[69][73]

Around 50% of women of childbearing age in developed countries like the United Kingdom are overweight or obese before pregnancy.[73] A systematic review found that diet is the most effective way to reduce weight gain and associated risks in pregnancy.[73] The review did not find evidence of harm associated with diet control and exercise.[73]

Medication use



Use of recreational drugs

Use of recreational drugs in pregnancy can cause various pregnancy complications.

Exposure to environmental toxins


Sexual activity

Most women can continue to engage in sexual activity throughout pregnancy.[82] Most research suggests that during pregnancy both sexual desire and frequency of sexual relations decrease.[83][84] In context of this overall decrease in desire, some studies indicate a second-trimester increase, preceding a decrease during the third trimester.[85][86] Some individuals are sexually attracted to pregnant women (pregnancy fetishism, also known as maiesiophilia).



Regular aerobic exercise during pregnancy appears to improve (or maintain) physical fitness; however, the quality of the research is poor and the data was insufficient to infer important risks or benefits for the mother or infant.[88] Physical exercise during pregnancy does appear to decrease the risk of C-section.[89]




It has been suggested that shift work and exposure to bright light at night should be avoided at least during the last trimester of pregnancy to decrease the risk of psychological and behavioral problems in the newborn.[91] A proposed underlying mechanism is that the circadian rhythm of the mother programs the developing rhythm of the fetus.[91]

Symptoms and discomforts

Melasma pigment changes to the face due to pregnancy


Common symptoms and discomforts of pregnancy include:



The following are some examples of pregnancy complications:


Intercurrent diseases






Society and culture

In most cultures, pregnant women have a special status in society and receive particularly gentle care.[100] At the same time, they are subject to expectations that may exert great psychological pressure, such as having to produce a son and heir. In many traditional societies, pregnancy must be preceded by marriage, on pain of ostracism of mother and (illegitimate) child.








Main article: Infertility



Main article: Abortion


Legal protection


In 2014, the American state of Kentucky passed a law which allows prosecutors to charge a woman with criminal assault if she uses illegal drugs during her pregnancy and her fetus or newborn is considered harmed as a result.[102]


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Further reading

External links