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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]


Pregnancy is the carrying of one or more offspring, known as a fetus or embryo, inside the body of a female mammal such as a human. 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, and the medical field that deals with pregnancy is called obstetrics.

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.

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.

Determining the Start of Pregnancy and Predicting Date of Birth

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" (though the definition of the English word "conception" is somewhat controversial). 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 (EDD).

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 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 EDD, although improvements to Naegele's rule have been suggested.[1] 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. 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.

EDD may also be calculated from sonogram measurement of the fetus. This method is slightly more accurate than methods based on LMP.[2] 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.[3]

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.

Medical signs

A number of medical signs are associated with pregnancy.[4] [5]

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.

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


Physical symptoms of pregnancy vary. Of the symptoms listed, not all will occur for every woman, and individuals may well experience different symptoms during different pregnancies. The following is a list of the most common symptoms.[4]

First trimester

Other symptoms may be experienced specifically during the later stages, such as:

  • Lower backache. Balance and ease of walking may be affected.
  • Many women will get flatulent and gassy.
  • Some may have difficulty in walking and balance.
  • Some women may experience haemorrhoids and rectal irritation.
  • Some women report hair loss, others have more body or "facial" hair.
  • Sensitivity in teeth, higher risk for gum disease.

Contact lens/spectacle prescriptions may be affected.

Some women during pregnancy experience mental disturbances more severe than typical mood swings. Psychological stress during pregnancy is associated with an increase in other pregnancy symptoms.[6]

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 with love partners 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.


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."


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.

Timeline of a Typical Pregnancy

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

Comparison of growth of the fetus between 26 weeks and 40 weeks 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 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.

A pregnant woman at 26 weeks

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. 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

Template:Seealso 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,[7] 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.[8] Brain stem activity has been detected as early as 54 days after conception,[9] and the first measurable signs of EEG activity occur in the 12th week.[10] Some fingerprint formation occurs from the beginning of the fetal stage.[11]

Food and Nutrition During Pregnancy

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. 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.

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.[16]

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).

Medical Aspects of Pregnancy

Diagnostic criteria are: Women who has a 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 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.[17] Omega 3 fatty acids support the mental and visual development of infants and they are also beneficial postpartum.[18]


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

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

Medical Disorders in Pregnancy

Most potentially serious problems 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.[19] A small percentage of pregnant women and new mothers have AIDS, cancer, heart disease, lung disease, physical disabilities, diabetes and psychiatric disorders like depression.

Two decades ago, many medical disorders were contra-indications to pregnancy. Advances in obstetrics, neonatology, obstetric anesthesiology, midwifery and medicine have increased the expectation that pregnancy will result in an excellent outcome for both the woman and the fetus, despite most of these conditions. Certain medications deemed harmless to the fetus may be highly necessary to the welfare and benefit of the woman, especially if she is pregnant, as some conditions are aggravated by pregnancy.

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, due to frequent vomiting.

A common yet under-diaganosed syndrome is "Pelvic girdle pain" (PGP). One in 35[20] women will experience some degree of PGP during their pregnancy. Symptoms can begin as early 12 weeks into the pregnancy. This muscloskeletal syndrome can effect any of the three pelvic joints. The female experiences a reduced tolerance for weightbearing activities ranging from being midly to severely disabled.

Pregnancy must be differentiated from other causes of irregular menses and hirsutism.

Disease Differentiating Features
  • Pregnancy always should be excluded in a patient with a history of amenorrhea
  • Features include amenorrhea or oligomenorrhea, abnormal uterine bleeding, nausea/vomiting, cravings, weight gain (although not in the early stages and not if vomiting), polyuria, abdominal cramps and constipation, fatigue, dizziness/lightheadedness, and increased pigmentation (moles, nipples)
  • Uterine enlargement is detectable on abdominal examination at approximately 14 weeks of gestation
  • Ectopic pregnancy may cause oligomenorrhea, amenorrhea, or abnormal uterine bleeding with abdominal pain and sometimes subtle or absent physical symptoms and signs of pregnancy
Hypothalamic amenorrhea
  • Diagnosis of exclusion
  • Seen in athletes, people on crash diets, patients with significant systemic illness, and those experiencing undue stress or anxiety
  • Predisposing features are as follows weight loss, particularly if features of anorexia nervosa are present or the BMI is <19 kg/m2
  • Recent administration of depot medroxyprogesterone, which may suppress ovarian activity for 6 months to a year
  • Use of dopamine agonists (eg, antidepressants) and major tranquilizers
  • Hyperthyroidism
  • In patients with weight loss related to anorexia nervosa, fine hair growth (lanugo) may occur all over the body, but it differs from hirsutism in its fineness and wide distribution
Primary amenorrhea
  • Causes include reproductive system abnormalities, chromosomal abnormalities, or delayed puberty
  • If secondary sexual characteristics are present, an anatomic abnormality (eg, imperforate hymen, which is rare) should be considered
  • If secondary sexual characteristics are absent, a chromosomal abnormality (eg, Turner syndrome ) or delayed puberty should be considered
Cushing syndrome
  • Cushing syndrome is due to excessive glucocorticoid secretion from the adrenal glands, either primarily or secondary to stimulation from pituitary or ectopic hormones; can also be caused by exogenous steroid use
  • Features include hypertension, weight gain (central distribution), acne, and abdominal striae Patients have low plasma sodium levels and elevated plasma cortisol levels on dexamethasone suppression testing
  • Mild hyperprolactinemia may occur as part of PCOS-related hormonal dysfunction
  • Other causes include stress, lactation, and use of dopamine antagonists
  • A prolactinoma of the pituitary gland is an uncommon cause and should be suspected if prolactin levels are very high (>200 ng/mL)
  • Physical examination findings are usually normal
  • As in patients with PCOS, hyperprolactinemia may be associated with mild galactorrhea and oligomenorrhea or amenorrhea; however, galactorrhea also can occur with nipple stimulation and/or stress when prolactin levels are within normal ranges
  • A large prolactinoma may cause headaches and visual field disturbance due to pressure on the optic chiasm, classically a gradually increasing bi-temporal hemianopsia
Ovarian or adrenal tumor
  • Benign ovarian tumors and ovarian cancer are rare causes of excessive androgen secretion; adrenocortical tumors also can increase the production of sex hormones
  • Abdominal swelling or mass, abdominal pain due to fluid leakage or torsion, dyspareunia, abdominal ascites, and features of metastatic disease may be present
  • Features of androgenization include hirsutism, weight gain, oligomenorrhea or amenorrhea, acne, clitoral hypertrophy, deepening of the voice, and high serum androgen (eg, testosterone, other androgens) levels
  • In patients with an androgen-secreting tumor, serum testosterone is not suppressed by dexamethasone
Congenital adrenal hyperplasia
  • Congenital adrenal hyperplasia is a rare genetic condition resulting from 21-hydroxylase deficiency
  • The late-onset form presents at or around menarche Patients have features of androgenization and subfertility
  • Affects approximately 1% of hirsute patients More common in Ashkenazi Jews (19%), inhabitants of the former Yugoslavia (12%), and Italians (6%)
  • Associated with high levels of 17-hydroxyprogesterone
  • A short adrenocorticotropic hormone stimulation test with measurement of serum17-hydroxyprogesterone confirms the diagnosis Assays of a variety of androgenic hormones help define other rare adrenal enzyme deficiencies, which present similarly to 21-hydroxylase deficiency
Anabolic steroid abuse
  • Anabolic steroids are synthetic hormones that imitate the actions of testosterone by increasing muscle bulk and strength
  • Should be considered if the patient is a serious sportswoman or bodybuilder
  • Features include virilization (including acne and hirsutism), often increased muscle bulk in male pattern, oligomenorrhea or amenorrhea, clitoromegaly, gastritis, hepatic enlargement, alopecia, and aggression
  • Altered liver function test results are seen
  • Hirsutism is excessive facial and body hair, usually coarse and in a male pattern of distribution
  • Approximately 10% of women report unwanted facial hair
  • There is often a family history and typically some Mediterranean or Middle Eastern ancestry
  • May also result from use of certain medications, both androgens, and others including danazol, glucocorticoids, cyclosporine, and phenytoin
  • Menstrual history is normal
  • When the cause is genetic, the excessive hair, especially on the face (upper lip), is present throughout adulthood, and there is no virilization
  • When secondary to medications, the excessive hair is of new onset, and other features of virilization, such as acne and deepened voice, may be present

Terms and Definitions


Conceptus between time of fertilization to 10 weeks of gestation.
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 the woman has been pregnant.
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.
Minimum age for fetus survival, ca. third trimester.
From fertilization until second cell division.
Full term
Refers to the end of 36 weeks (nine months) from the first day of the woman'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, ranging from polite ("mother to be", "with child", "expecting" and "full") to crude ("up the duff" - UK, "knocked up" and "preggo" - US, and "down the well" - Australia), and even comical ("bun in the oven").

Regional Customs

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

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.

Related Chapters



  1. Mittendorf R, Williams MA, Berkey CS, Cotter PF. The length of uncomplicated human gestation. Obstet Gynecol 1990;75:929-32. PMID 2342739.
  2. Nguyen, T.H. (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. Unknown parameter |coauthors= ignored (help)
  3. Odutayo, Rotimi (n.d.). "Post Term Pregnancy". Unknown parameter |coauthors= ignored (help)
  4. 4.0 4.1 "Early symptoms of pregnancy: What happens right away". Mayo Clinic. February 22, 2007.
  5. American Pregnancy Association, Pregnancy Symptoms. Retrieved 2007-09-15.
  6. Rodriguez, Alina (2001). "Symptoms across pregnancy in relation to psychosocial and biomedical factors". Acta Obstetricia et Gynecologica Scandinavica. 80 (3): pp. 213-233. Unknown parameter |coauthors= ignored (help)
  7. 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.
  8. 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.
  9. Singer, Peter. Rethinking life & death: the collapse of our traditional ethics, page 104 (St. Martins Press 1996). Retrieved 2007-03-04.
  10. 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.
  11. Zabinski, Mark. Forensic Series Seminar, Pastore Chemical Laboratory, University of Rhode Island (February 2003) (news report retrieved 2007-01-20).
  12. 3D Pregnancy (Image from gestational age of 6 weeks). Retrieved 2007-08-28. A rotatable 3D version of this photo is available here, and a sketch is available here.
  13. 3D Pregnancy (Image from gestational age of 10 weeks). Retrieved 2007-08-28. A rotatable 3D version of this photo is available here, and a sketch is available here.
  14. 3D Pregnancy (Image from gestational age of 20 weeks). Retrieved 2007-08-28. A rotatable 3D version of this photo is available here, and a sketch is available here.
  15. 3D Pregnancy (Image from gestational age of 40 weeks). Retrieved 2007-08-28. A rotatable 3D version of this photo is available here, and a sketch is available here.
  16. (Pregnancy and Diet; "Omega 3 Rich Foods and Pregnancy")
  17. 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).
  18. Douaud, Clarisse. "Omega-3 least known of pregnancy 'Big 3'", (2006-09-18).
  19. American Pregnancy Association, “Labor and Birth”. Retrieved 2007-09-15.
  20. Symphysis Pubis Dysfunction--A Cause of Significant Obstetric Morbidity. Eur J Obstet Gynecol Reprod Biol. 2002 Nov 15;105(2):143-6.Owens K, Pearson A, Mason

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