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  Pregnancy 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 medical field that studies and treats pregnant patients.

Childbirth usually occurs about 38 weeks from fertilization, i.e., approximately 40 weeks from the start of the last menstruation. Thus, pregnancy lasts about nine 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. The term embryo is used to describe the developing human during the initial weeks, and the term fetus is used from about two months of development until birth. A woman who is pregnant for the first time is known medically as a primigravida or "gravida 1", while a woman who has never been pregnant is known as "gravida 0". Similarly, the terms "para 0", "para 1" and so on are used for the number of times a woman has given birth. If a woman has never given birth, she is referred to as nulliparous.[1]

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.



Before pregnancy begins, a female oocyte (egg) must join with a spermatozoon in a process referred to in medicine as "fertilization", or commonly known as "conception". Fertilization usually occurs through the act of sexual intercourse, in which a spermatozoon penetrates and fertilizes an egg. However, the advent of artificial insemination has made it possible for women to become pregnant if pre-existing medical conditions in either the woman or the man make fertilization through sexual intercourse difficult, or if a woman chooses to become pregnant without a male partner. Though pregnancy begins at implantation, it is often convenient to date from the first day of a woman's last menstrual period, sometimes abbreviated "LMP". This is used to calculate the expected date of delivery.

Traditionally a human pregnancy is considered to last approximately 40 weeks (280 days) from the LMP, or 38 weeks (266 days) from the date of fertilization. The 38 weeks of gestation is nearly 10 lunar months, i.e. 27.3 days/lunar-month x 10 = 273 days. In the more familiar Gregorian calendar, the 40 weeks dating from the LMP is equivalent to a little more than nine months and six days. This forms the basis of Naegele's rule of approximating the expected date of delivery, although improvements to Naegele's rule have been suggested.[2] 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 postmature.

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. [3] The due date is typically calculated as 40 weeks from the last menstrual period.

An accurate date of fertilization 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 labor if a fetus is perceived to be overdue. Due dates are only a rough estimate, and the process of accurately dating a pregnancy is complicated by the fact that not all women have 28 day menstrual cycles, or ovulate on the 14th day following their last menstrual period.

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.[4] Confinement, the beginning of labor, begins on the day predicted by LMP 3.6% of the time and on the day predicted by sonography 4.3% of the time.[5]

Medical signs

A number of medical signs are associated with pregnancy.[6] [7]

Early signs

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 may be useful to make a presumptive diagnosis of pregnancy.

  • Presence of human chorionic gonadotropin (hCG) in the blood and urine, detectable by laboratory or home testing; this is the most reliable early sign of pregnancy
  • Missed menstrual period
  • Implantation bleeding, light spotting that occurs at implantation of the embryo in the uterus, in the third or fourth week after LMP
  • Increased basal body temperature sustained for over two weeks after ovulation
  • Chadwick's sign, a darkening of the cervix, vagina, and vulva
  • Goodell's sign, a softening of the vaginal portion of the cervix
  • Hegar's sign, a softening of the cervical isthmus

Later signs

  • Linea nigra, a darkening of the skin in a vertical line on the abdomen, caused by hyperpigmentation resulting from hormonal changes; it usually appears around the middle of pregnancy
  • Steadily increasing abdominal swelling, the most visible sign of pregnancy


Main article: Obstetrics

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, 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.

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.


The early stages of pregnancy are often discovered by using a pregnancy test, as soon as 48 hours after fertilization using sophisticated testing methods, but not until six to twelve days after fertilization using more typical methods. Pregnancy tests typically detect the presence of human chorionic gonadotropin.


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

Pregnancy is typically broken into three periods, or trimesters, each of about three months. While there are no hard and fast rules, these distinctions are useful in describing the changes that take place over time.

First trimester


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 fetal development.

Morning sickness afflicts about seventy percent of all pregnant women, typically only in the first trimester.[citation needed] 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. Although the fetus begins moving and takes a recognizable human shape during the first trimester, it is not until the second trimester that movement of the fetus, often referred to as "quickening", can be felt. 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's 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 woman's navel will sometimes become convex, "popping" out, due to her expanding abdomen. This period of her pregnancy can be uncomfortable, causing symptoms like weak bladder control and back-ache. Movement of the fetus becomes stronger and more frequent and via improved brain, eye, and muscle function the fetus is prepared for ex utero viability. The woman can feel the fetus "rolling" and it may cause pain or discomfort when it is near the woman's ribs.

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[citation needed]. 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.

Prenatal development and sonograph images

See also: Prenatal Development

Prenatal development is divided into two primary biological stages. The first is the embryonic stage, which lasts for about two months. At this point, the fetal stage begins. At the beginning of the fetal stage, the risk of miscarriage decreases sharply,[8] all major structures including hands, feet, head, brain, and other organs are present, and they continue to grow and develop. 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 sonograph; the fetus bends the head, and also makes general movements and startles that involve the whole body.[9] Brain stem activity has been detected as early as 54 days after conception,[10] and the first measurable signs of EEG activity occur in the 12th week.[11] Some fingerprint formation occurs from the beginning of the fetal stage.[12]


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 and 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 a rat's entire life.[13] Omega 3 fatty acids support the mental and visual development of infants and they are also beneficial postpartum.[14]


It is important for a pregnant woman to eat a healthy diet. Unless she has a specific health problem (e.g., diabetes mellitus or edema) balancing carbohydrates, fat, and proteins; and eating a variety of foods, including dairy products and several fruits and vegetables all contribute to a healthy pregnancy. A pregnant woman may choose to consult her obstetrician for specific advice. Some specific nutritional needs for pregnancy include:

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 fetus' neural tube. It thus helps prevent spina bifida, a very serious birth defect. Because neural tube development happens in the first 28 days of pregnancy, it is recommended that women take sufficient folic acid one month prior to becoming pregnant. 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 supplemented with folic acid.

Calcium and iron are particularly needed by the rapidly growing fetus. Pregnant women should eat enough dairy products (for calcium) and red meat (for iron) if they are not lactose intolerant or vegetarian. Women who do not eat dairy or meat can obtain calcium and iron from fortified soy milk and juice, soybeans, and certain leafy greens. Care providers may prescribe iron pills if pregnant women develop iron deficiency anemia. Calcium is effective only if women also obtain enough Vitamin D. The best way to get vitamin D is to sunbathe each day for 10–15 minutes. Salmon and fatty fishes are also good sources of vitamin D.

Fluoride is critical for development of teeth by increasing binding of calcium, strengthening the enamel. If water or salt does not contain fluoride, supplements are recommended during the third trimester. Many municipalities and water treatment plants add fluoride to the water.[citation needed]

Fat from salmon, trout, tuna, herring, sardine, mackerel, and some chicken eggs contain long-chain omega-3 (n-3) fatty acids that are needed to build neuron membranes. Thus fatty fish intake during pregnancy may provide nutrition for proper brain and retina development of the fetus. 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. Omega-3 fatty acids are also present in walnuts, flaxseed, and marine algae.[15]

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); try to avoid contact with cat feces (toxoplasma); clean the fridge often with diluted chlorine (then rinse).

Immunological Tolerance

Main article: Pre-eclampsia

Research on the immunological basis for pre-eclampsia has indicated that continued exposure to a partner's semen has a strong protective effect against pre-eclampsia, largely due to the absorption of several immune modulating factors present in seminal fluid.[16] Studies also showed that long periods of sexual cohabitation with the same partner fathering a woman's child significantly decreased her chances of suffering pre-eclampsia.[17] Several other studies have since investigated the strongly decreased incidence of pre-eclampsia in women who had received blood transfusions from their partner, those with long, preceding histories of sex without barrier contraceptives, and in women who had been regularly performing oral sex,[18] with one study concluding that "induction of allogeneic tolerance to the paternal HLA molecules of the fetus may be crucial. Data collected strongly suggests that exposure, and especially oral exposure to soluble HLA from semen can lead to transplantation tolerance."[18]

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,"[19] 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.[20] [21] 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. [22] 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.[22] Recognizing that some of the studies potentially included the presence of confounding factors, such as the likelihood that women who regularly perform oral sex and swallow semen engage in more frequent vaginal and anal 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.

When reporting the findings of the research group mentioned above, New Scientist magazine thought it was worth mentioning that many of the members on the research team were women (including the lead author).

Sexuality during pregnancy

Finally, one popular but exaggerated symptom is an increase in sexual urgency, mostly between the 5th and 8th month, when hormonal changes and physical discomfort such as nausea disappear. Some pregnant women reported a decrease in sexual interest, due to prolonged discomfort or psychological aspects such as not feeling "attractive" to pursue sexual activity, meaning that being pregnant affects their self-sense of beauty.

Until the mid 20th century, it was considered a socio-moral "taboo" action for pregnant women to engage in sexual activities. However, that perception has changed today. Sex during pregnancy is a low-risk behavior 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 labor 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. This is because studies find that those who have sex more than once a week feel less emotionally tense and more confident in the perception of their appearances. Some studies also suggest that they have less painful or quickened labors, which some physicians believe might be because sexual intercourse serves as a type of pelvic exercise.[citation needed]



See also: Complications of pregnancy

The following complications may occur during of pregnancy:

  • Back pain. A common complication, particularly in the third trimester when the patient's center of gravity has shifted.
  • Constipation. Caused by decreased bowel motility secondary to elevated progesterone (normal in pregnancy), which can lead to greater absorption of water.
  • Contractions. Occasional, irregular, painless contractions that occur several times per day are normal and are known as Braxton Hicks contractions. These contractions are usually caused by moderate or severe dehydration.
  • Dehydration. Caused by expanded intravascular space and increased Third Spacing of Fluids
  • Edema. Caused by compression of the inferior vena cava (IVC) and pelvic veins by the uterus leads to increased hydrostatic pressure in lower extremities.
  • Gastroesophageal Reflux Disease (GERD). Caused by: relaxation of the lower esophageal sphincter (LES) and increased transit time in the stomach (normal in pregnancy)
  • Hemorrhoids. Caused by increased venous stasis and IVC compression leading to congestion in venous system along with increased abdominal pressure secondary to constipation.
  • Pica. Unexplained cravings for nonedible items such as dirt or clay. Commonly, patients will be placed on ice chips to chew on instead of these nonedible items.
  • Lower abdominal pain. Caused by rapid expansion of the uterus and stretching of ligaments such as the round ligament.
  • Increased urinary frequency. Caused by increased intravascular volume, elevated GFR (glomerular filtration rate), and compression of the bladder by the expanding uterus.
  • Varicose veins. Caused by relaxation of the venous smooth muscle and increased intravascular pressure.


Main article: Abortion

An abortion is the removal or expulsion of an embryo or fetus from the uterus, resulting in or caused by its death. This can occur spontaneously or accidentally as with a miscarriage, or be artificially induced by medical, surgical or other means.


Main article: Childbirth

Childbirth is the process by which an infant 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 labor 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 labors, while others find that concentrating on the birth helps to quicken labor and lessen the sensations. Most births are successful vaginal births, but sometimes complications arise and a woman may undergo a caesarean section. While some caesarean sections are necessary, as in the case of cord prolapse, there is dispute as to whether the procedure is used too frequently. Some women choose to have a caesarean section.

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

Postnatal period

Main article: Postnatal

Cultural references

In Korea, China, and Vietnam, age is measured starting from conception to acknowledge that the fetus exists within the woman's body before it is born.[citation needed] Therefore, a newborn baby is considered to be one year old, although gestation is approximately 9 months.[citation needed]

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 fetus develops.[citation needed]

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. 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 use the word "baby."[citation needed]

See also


  1. ^ Retrieved 2007-12-27.
  2. ^ Mittendorf R, Williams MA, Berkey CS, Cotter PF. The length of uncomplicated human gestation. Obstet Gynecol 1990;75:929-32. PMID 2342739.
  3. ^ Dr Sally Tracy, Having a Great Birth in Australia ed. David Vernon, Australian College of Midwives, 2005, p22
  4. ^ Nguyen, T.H.; et al. (1999). "Evaluation of ultrasound-estimated date of delivery in 17 450 spontaneous singleton births: do we need to modify Naegele's rule?". Ultrasound in Obstetrics and Gynecology 14 (1): 23-28. Retrieved on 2007-08-18.
  5. ^ Odutayo, Rotimi; Odunsi, Kunle (n.d.). Post Term Pregnancy. Retrieved on 2007-08-18.
  6. ^ Early symptoms of pregnancy: What happens right away. Mayo Clinic (February 22, 2007). Retrieved on 2007-08-22.
  7. ^ American Pregnancy Association, Pregnancy Symptoms. Retrieved 2007-09-15.
  8. ^ • Q&A: Miscarriage. (August 6 , 2002). BBC News. Retrieved 2007-04-22: “The risk of miscarriage lessens as the pregnancy progresses. It decreases dramatically after the 8th week.”
    • Lennart Nilsson, A Child is Born 91 (1990): at eight weeks, "the danger of a miscarriage … diminishes sharply."
    • “Women’s Health Information”, Hearthstone Communications Limited: “The risk of miscarriage decreases dramatically after the 8th week as the weeks go by.” Retrieved 2007-04-22.
  9. ^ 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.
  10. ^ Singer, Peter. Rethinking life & death: the collapse of our traditional ethics, page 104 (St. Martins Press 1996). Retrieved 2007-03-04.
  11. ^ Vogel, Friedrich. Genetics and the Electroencephalogram (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 2007-07-24.
  12. ^ Zabinski, Mark. Forensic Series Seminar, Pastore Chemical Laboratory, University of Rhode Island (February 2003) (news report retrieved 2007-01-20).
  13. ^ Tees, R and Mohammadi, E. "The effects of neonatal choline dietary supplementation on adult spatial and configural learning and memory in rats", Developmental Psychobiology (November 1999).
  14. ^ Douaud, Clarisse. "Omega-3 least known of pregnancy 'Big 3'", (2006-09-18).
  15. ^ (Pregnancy and Diet; "Omega 3 Rich Foods and Pregnancy")
  16. ^ Sarah Robertson. Research Goals.
  17. ^ Einarsson, Jon I. MD; Sangi-Haghpeykar, Haleh PhD; Gardner, Michael O. MD, MPH (2003). "Sperm exposure and development of preeclampsia". Journal of Obstetrics and Gynecology 188 (5): 1241-1243.
  18. ^ a b Koelman CA, Coumans AB, Nijman HW, Doxiadis II, Dekker GA, Claas FH (2000). "Correlation between oral sex and a low incidence of preeclampsia: a role for soluble HLA in seminal fluid?". J. Reprod. Immunol. 46 (2): 155–66. PMID 10706945.
  19. ^ Martina Johansson, John J Bromfield, Melinda J Jasper, and Sarah A Robertson (2004). "Semen activates the female immune response during early pregnancy in mice". Journal of Immunology 112 (2): 290-300.
  20. ^ Burne, Jerome. "Give Sperm a Fighting Chance", The Times, 2006-01-30. Retrieved on 2007-11-16. 
  21. ^ "Sex Primes Women for Sperm", BBC News, 2002-02-06. Retrieved on 2007-11-19. 
  22. ^ a b Fox, Douglas. "Gentle Persuasion", The New Scientist, 2002-02-09. Retrieved on 2007-06-17. 

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