Pregnancy (latin graviditas) is the carrying of one or more offspring, known as a fetus or embryo, inside the uterus of a female human. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets. Human pregnancy is the most studied of all mammalian pregnancies. Obstetrics is the surgical field that studies and treats pregnancy. Midwifery is the non-surgical field that cares for pregnant women.
Childbirth usually occurs about 38 weeks from fertilization (conception), i.e., approximately 40 weeks from the the last normal menstrual period (LNMP) in humans. The date of delivery is considered normal medically if it falls within two weeks of the calculated date. The calculation of this date involves the assumption of a regular 28-day period. Thus, pregnancy lasts almost ten months, although the exact definition of the English word “pregnancy” is a subject of controversy.
One scientific term for the state of pregnancy is gravid, and a pregnant female is sometimes referred to as a gravida. Both words are rarely used in common speech. Similarly, the term "parity" (abbreviated as "para") is used for the number of previous successful live births. Medically, a woman who is not currently pregnant or who has never been pregnant is referred to as a "nulligravida", and in subsequent pregnancies as "multigravida" or "multiparous". Hence during a second pregnancy a woman would be described as "gravida 2, para 1" and upon delivery as "gravida 2, para 2". Incomplete pregnancies of abortions, miscarriages or stillbirths account for parity values being less than the gravida number, whereas a multiple birth will increase the parity value. Women who have never carried a pregnancy achieving more than 20 weeks of gestation age are referred to as "nulliparous".
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 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 beginning of the third trimester often approximates the point of viability, or the ability of the fetus to survive, with or without medical help, outside of the uterus.
Pregnancy occurs as the result of the female gamete or oocyte (egg) being penetrated by the male gamete spermatozoon in a process referred to, in medicine, as "fertilization", or more commonly known as "conception". The fusion of male and female gametes usually occurs through the act of sexual intercourse. However, the advent of artificial insemination has also made achieving pregnancy possible in such cases where sexual intercourse is not potentially fertile (through choice or male/female infertility).
A number of medical signs are associated with pregnancy. 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 two 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; it usually appears around the middle of pregnancy).
Pregnancy is considered 'at term' when gestation attains 37 complete weeks but is less than 42 (between 259 and 294 days since LMP). Events before completion of 37 weeks (259 days) are considered pre-term; from week 42 (294 days) events are considered post-term. When a pregnancy exceeds 42 weeks (294 days), the risk of complications for mother and fetus increases significantly. As such, obstetricians usually prefer to induce labour, in an uncomplicated pregnancy, at some stage between 41 and 42 weeks.
Recent medical literature prefers the terminology pre-term and post-term to premature and post-mature. Pre-term and post-term are unambiguously defined as above, whereas premature and postmature have historical meaning and relate more to the infant's size and state of development rather than to the stage of pregnancy.
Fewer than 5% of births occur on the due date; 50% of births are within a week of the due date, and almost 90% within two weeks. It is much more useful to consider therefore a range of due dates, rather than one specific day, with some online due date calculators providing this information, e.g. here
Accurate dating of pregnancy is important, because it is used in calculating the results of various prenatal tests (for example, in the triple test). A decision may be made to induce labour if a fetus is perceived to be overdue. Furthermore, if a difference of predicted due date is highlighted between dates based on LMP and ultrasound dating, with the latter being later, this might signify slowed fetal growth and therefore the need for closer review.
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.
An early sonograph 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, or she has been charting her cycles, or the conception is as the result of some types of fertility treatment (such as IUI or IVF) 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 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. The expected date of delivery may also be calculated from sonogram measurement of the fetus. This method is slightly more accurate than methods based on LMP. The beginning of labour, which is variously called confinement or childbed, begins on the day predicted by LMP 3.6% of the time and on the day predicted by sonography 4.3% of the time.
Diagnostic criteria are: Women who have menstrual cycles and are 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.
Traditionally, doctors have measured pregnancy from a number of convenient points, including the day of last menstruation, ovulation, fertilization, implantation and chemical detection. In medicine, pregnancy is often defined as beginning when the developing embryo becomes implanted into the endometrial lining of a woman's uterus. 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 foetal development.
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 surviving, 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.
One way to observe prenatal development is via ultrasound images. Modern 3D ultrasound images provide greater detail for prenatal diagnosis than the older 2D ultrasound technology. Whilst 3D is popular with parents desiring a prenatal photograph as a keepsake, both 2D and 3D are discouraged by the FDA for non-medical use, but there are no definitive studies linking ultrasound to any adverse medical effects. The following 3D ultrasound images were taken at different stages of pregnancy:
Prolactin levels increase due to maternal Pituitary gland enlargement by 50%. This mediates a change in the structure of the Mammary gland from ductal to lobular-alveolar. Parathyroid hormone is increased due to increases of calcium uptake in the gut and reabsorption by the kidney. Adrenal hormones such as cortisol and aldosterone also increase.
Placental lactogen is produced by the placenta and stimulates lipolysis and fatty acid metabolism by the mother, conserving blood glucose for use by the fetus. It also decreases maternal tissue sensitivity to insulin, resulting in gestational diabetes.
Cardiac function is also modified, with increase heart rate and increased stroke volume. A decrease in vagal tone and increase in sympathetic tone is the cause. Blood volume increases act to increase stroke volume of the heart via Starling's law. After pregnancy the change in stroke volume is not reversed. Cardiac output rises from 4 to 7 litres in the 2nd trimester
Blood pressure also fluctuates. In the first trimester it falls. Initially this is due to decreased sensitivity to angiotensin and vasodilation provoked by increased blood volume. Later, however, it is caused by decreased resistance to the growing uteroplacental bed.
Progesterone may act centrally on chemoreceptors to reset the set point to a lower partial pressure of carbon dioxide. This maintains an increased respiration rate even at a decreased level of carbon dioxide.
Maternal insulin resistance can lead to gestational diabetes. Increase liver metabolism is also seen, with increased gluconeogenesis to increase maternal glucose levels.
Adequate periconceptional folic acid (also called folate or Vitamin B9) intake has been proven to limit fetal neural tube defects, preventing spina bifida, a very serious birth defect. The neural tube develops during the first 28 days of pregnancy, explaining the necessity to guarantee adequate periconceptional folate intake. Folates (from folia, leaf) are abundant in spinach (fresh, frozen, or canned), and are also found in green vegetables, salads, melon, hummus, and eggs. In the United States and Canada, most wheat products (flour, noodles) are fortified with folic acid.
Several micronutrients are important for the health of the developing fetus, especially in areas of the world where insufficient nutrition is prevalent. 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.
There is some evidence that long-chain omega-3 (n-3) fatty acids have an effect on the developing fetus, but further research is required. At this time, supplementing the diet with foods rich in these fatty acids is not recommended, but is not harmful.
Dangerous bacteria or parasites may contaminate foods, particularly listeria and toxoplasma, toxoplasmosis agent. Careful washing of fruits and raw vegetables may remove these pathogens, as may thoroughly cooking leftovers, meat, or processed meat. Soft cheeses may contain listeria; if milk is raw the risk may increase. Cat feces pose a particular risk of toxoplasmosis. Pregnant women are also more prone to catching salmonella infections from eggs and poultry, which should be thoroughly cooked. Practicing good hygiene in the kitchen can reduce these risks.
Other studies have investigated the roles of semen in the female reproductive tracts of mice, showing that "insemination elicits inflammatory changes in female reproductive tissues," concluding that the changes "likely lead to immunological priming to paternal antigens or influence pregnancy outcomes." A similar series of studies confirmed the importance of immune modulation in female mice through the absorption of specific immune factors in semen, including TGF-Beta, lack of which is also being investigated as a cause of miscarriage in women and infertility in men.
According to the theory, pre-eclampsia is frequently caused by a failure of the mother's immune system to accept the fetus and placenta, which both contain "foreign" proteins from paternal genes. Regular exposure to the father's semen causes her immune system to develop tolerance to the paternal antigens, a process which is significantly supported by as many as 93 currently identified immune regulating factors in seminal fluid. Having already noted the importance of a woman's immunological tolerance to her baby's paternal genes, several Dutch reproductive biologists decided to take their research a step further. Consistent with the fact that human immune systems tolerate things better when they enter the body via the mouth, the Dutch researchers conducted a series of studies that confirmed a surprisingly strong correlation between a diminished incidence of pre-eclampsia and a woman's practice of oral sex, and noted that the protective effects were strongest if she swallowed her partner's semen. The researchers concluded that while any exposure to a partner's semen during sexual activity appears to decrease a woman's chances for the various immunological disorders that can occur during pregnancy, immunological tolerance could be most quickly established through oral introduction and gastrointestinal absorption of semen. Recognizing that some of the studies potentially included the presence of confounding factors, such as the possibility that women who regularly perform oral sex and swallow semen might also engage in more frequent vaginal intercourse, the researchers also noted that, either way, the data still overwhelmingly supports the main theory behind all their studies--that repeated exposure to semen establishes the maternal immunological tolerance necessary for a safe and successful pregnancy.
Sex during pregnancy is a low-risk behaviour except when the physician advises that sexual intercourse be avoided, which may, in some pregnancies, lead to serious pregnancy complications or health issues such as a high-risk for premature labour or a ruptured uterus. Such a decision may be based upon a history of difficulties in a previous childbirth.
Some psychological research studies in the 1980s and '90s contend that it is useful for pregnant women to continue to have sexual activity, specifically noting that overall sexual satisfaction was correlated with feeling happy about being pregnant, feeling more attractive in late pregnancy than before pregnancy and experiencing orgasm. Sexual activity has also been suggested as a way to prepare for induced labour; some believe the natural prostaglandin content of seminal liquid can favour the maturation process of the cervix making it more flexible, allowing for easier and faster dilation and effacement of the cervix. However, the efficacy of using sexual intercourse as an induction agent "remains uncertain".
During pregnancy, the baby is protected from penetrative thrusting by the amniotic fluid in the womb and by the woman's abdomen.
The following are complaints that may occur during pregnancy:
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 labour and lessen the sensations. Most births are successful vaginal births, but sometimes complications arise and a woman may undergo a caesarean section.
In the context of political debates regarding a proper definition of life, the terminology of pregnancy can be confusing. 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. De Crespigny observes that doctors' language has a powerful influence over the way patients think, and thus proposes that the best interests of patients are served by using language that both supports patient autonomy and is neutral.