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Pregnancy is the condition of having a developing embryo or fetus in the body. The union of an egg (ovum) with sperm is called fertilization, or conception, and it is this union that produces the embryo. Pregnancy includes the period from conception to birth of the fetus, and usually lasts 10 lunar months (40 weeks/280 days), or nine calendar months—as measured from the first day of the last menstrual period (LMP). It is also referred to as a gestation period that consists of three trimesters. The trimesters are not equal, but are defined by different stages of a baby's development. The first trimester includes the first 13 weeks of pregnancy, or the first 12 weeks of life. The second trimester consists of weeks 14-26, and the third trimester is weeks 27-40.


At the time of sexual intercourse, a man ejects millions of sperm into the woman's vagina. The sperm travel in all directions, propelled by their whiplike tails, and many swim through the cervix toward the uterus. A very small number of them may survive as long as 48 hours, but only one has to make it to the fallopian tube to meet the egg that has been released from an ovary. It takes approximately 30 minutes following intercourse for the sperm to meet the egg. One sperm penetrates the egg and its tail is shed, while the remainder of the sperm provides one-half of the genetic material of the future fetus—the other half provided by the egg. The fertilized egg then travels along the fallopian tube, arriving in the uterus four to five days later. Fluid secreted by the tube lining provides it with nutrition during its travels. After two to three days in the uterine cavity, the fertilized egg implants into the thick lining of the uterus. Implantation occurs at about day 22 of a normal menstrual cycle. If the fertilized egg were visible to the naked eye, it would appear to be covered with fine hairs, called villi. These villi become the densest where the blood supply is richest, and eventually form the placenta. The mother's blood moves slowly around these villi, permitting them to absorb food and oxygen, and to eliminate waste products. The placenta is completely formed and functioning by 10 weeks after fertilization. Between 12 and 20 weeks' gestation, the placenta weighs more than the fetus, because the fetal organs are not sufficiently developed to deal with the processes needed for nutrition.

First month

At week four of pregnancy, the embryo is about one-eighth of an inch (0.275 cm) long, and weighs about 1/32 of an ounce (3 g). A formed yolk sac is present.

Second month

During the next four weeks, the embryo will grow to be about one inch long (2.5 cm) and weigh about 5/16 of an ounce (8.7 g). The umbilical cord will form, and the pulsation of the heart can be noted. The head and tail of the embryo are formed, and sex glands are determined, although the external genitals cannot be visibly identified as male or female. Limbs are well formed, and toes and fingers are present. The development of a skeleton and the formation of bone cells begin. Facial features begin to form, as does the external portion of the ear. The eyelids are fused throughout this period. By this stage, the fetus has a distinctly human appearance, and the beginnings of all the main organ systems are established. Since the structures of the brain, heart, liver, limbs, ears, nose, and eyes develop by the end of eight weeks, this is considered the most critical period of development. Any exposure to medications, alcohol, or illicit drugs during this time may cause defects, or anomalies, in the fetus.

Third month

Approximately nine weeks after conception, the baby has developed the features of a human being, and is called a fetus, not an embryo. Limb movements first occur at the end of the embryo stage, although they are not coordinated and cannot be felt. At 12 weeks of pregnancy, the fetus is 1-3 inches (7.5 cm) long from head to heel, and weighs about one ounce (28 g). The formation of red blood cells has already occurred in the liver, but now the spleen takes over making them. Urine formation begins between the ninth and twelfth weeks, and is discharged into the amniotic fluid. The fetus can reabsorb some of this fluid after swallowing it. Waste products are now transferred to the mother's circulatory system by crossing the placenta.

Fourth month

Growth is very rapid during this period, and limb movements become coordinated, although it is difficult for the mother to feel them. An ultrasound reveals the bones of the fetal skeleton, which are clearly visible. Their development continues as the limbs lengthen. Scalp hair patterning is also determined during this period, and slow eye movements can occur at about 14 weeks. External genitals can be recognized by 14 weeks, and the external ears stand out from the head. The fetus is now about 6 inches (15 cm) long and weighs about 4 ounces (112 g).

Fifth month

"Quickening" is the mother's feeling the baby move for the first time; it usually occurs during this period. The average time between a mother's first detection of fetal movements and delivery is 147 days, with a deviation between plus or minus 15 days. The baby's skin is now covered with a greasy, cheese-like material called "vernix caseosa," and it protects the delicate fetal skin from cuts, chapping, and hardening, all of which could occur from exposure to the amniotic fluid. Eyebrows and head hair are also visible at 20 weeks, and the fetus is usually completely covered with fine, downy hair (the lanugo), which helps to hold the vernix on the skin. Brown fat forms during this period to prepare for heat production when the baby is born. By 18 weeks, a female fetus has a formed uterus, and the opening for the vagina has begun. Many egg-forming follicles are also forming in the ovaries. By 20 weeks in a male fetus, the testes have begun to descend, but they are still located inside the abdominal wall. The fetus now weighs about one pound (454 g) and is 10 inches (25 cm) long.

Sixth month

There is a substantial weight gain this month. The skin, usually wrinkled, appears clear, and is pink to red because blood is visible in the capillaries. At 21 weeks, rapid eye movements (REMs) begin, and blink-startle responses are visible on ultrasound following a loud noise. Fingernails are present by 24 weeks, and the cells in the lung have begun to secrete a substance necessary to develop the alveoli of the lungs. In most medical practices, a fetus born before 24 weeks is not considered viable or capable of living, but if born at 24 weeks, attempts will be made for survival. The chances of a good outcome are, however, very poor. The fetus now weighs about 1.5 pounds (730 g), and is about 13 inches (32 cm) long.

Seventh month

By 26 weeks, the eyes are partially open and eyelashes are present. At 28 weeks, the eyes are wide open and a good head of hair is often present. At this age, a fetus can often survive even if born prematurely, presuming it is given intensive care. The lungs and blood circulation are developed, and can provide a better exchange of oxygen. Also, the central nervous system is now more mature, and can manage rhythmic breathing movements as well as assist in controlling body temperature. Toenails are present and more fat is deposited, smoothing the wrinkly skin. At 28 weeks, the bone marrow takes over the red blood cell-making work of the spleen, becoming the major site of this process. At 30 weeks, a light reflex of the eyes can be obtained. The skin is pink and smooth, and the limbs have a chubby appearance. The fetus might weigh as much as 3 lbs (1.3 kg), and is about 14-15 inches (35-37.5 cm) long. The fetus can be observed on ultrasound; it is sucking its thumb and practicing breathing movements. The mother may experience hiccups as rhythmic movements when the baby is practicing its breathing.

Eighth month

Fetuses 32 weeks and older usually survive if born prematurely. At 32 weeks, the fingernails reach the fingertips. At 35 weeks, fetuses have a firm grasp and show a spontaneous orientation to light. Growth continues, but slows as the baby begins to take up most of the room in the uterus. Now weighing between 3.5-4.5 lbs (1.7-2.3 kg), and measuring 16-18 inches (40-45 cm) long, the fetus may prepare for delivery by moving into the head-down position.

Ninth month

At 36 weeks, the body appears plump. The hair covering the body is almost gone. Toenails reach toe tips and the limbs are flexed. A full-term baby is one born anywhere from 37-40 weeks' gestation. A baby born after 41 weeks is considered postdate. Adding 0.5 lb (227 g) a week as the due date approaches, the fetus drops lower into the mother's abdomen and prepares for the onset of labor, which may begin any time between the 37th and 41st week of gestation. The expected date of delivery (EDD) of a fetus is 266 days, or 38 weeks after fertilization (i.e., 280 days or 40 weeks after the LMP). Prolongation of pregnancy occurs in 5-6% of women. If the pregnancy continues past 41-42 weeks, the physician will order fetal monitoring to determine the status of the baby. Since the mortality rate increases significantly after two to three weeks postdate, labor is often induced. Most healthy babies will weigh 6-8 lbs (2.7-3.6 kg) at birth, and will be about 19-21 inches (47-52 cm) long.

Causes and symptoms

The first sign of pregnancy is usually a missed menstrual period. A little bleeding or spotting may occur, due to implantation of the fertilized egg. Some women experience no early symptoms of pregnancy during the first few weeks, while others may experience all of them. A woman's breasts usually seem larger and feel tender as the mammary glands prepare for eventual breast-feeding. Nipples begin to enlarge and the veins over the surface of the breasts become more noticeable. Morning sickness (i.e., nausea and vomiting) is somewhat common, and can happen at any time—day or night. Extreme sensitivity to smell may worsen nausea. It is normal to feel bloated and more tired than usual. Frequent urination is common, and the pregnant woman may find herself getting up during the night to urinate. There may be a creamy white discharge from the vagina; that is normal. Food cravings may occur. Most women gain about 2-4 pounds (0.7-1.8 kg) by the end of the first trimester (0-13 weeks), and their clothes begin to feel tight.

In the second trimester (14-26 weeks), morning sickness usually ends and a woman's appetite may increase. There is a weight gain of about 12-15 pounds (5.4-6.75 kg) during this trimester. Most women begin to look pregnant and feel more energetic. Heart rate increases, as does the volume of blood in the body. This may cause a woman to feel flushed and warm at times. Some women experience constipation, heart-burn and indigestion, backache, sleeplessness, or swollen feet during the second trimester. Physical activity, such as swimming and walking, will help constipation as well as drinking plenty of fluids (i.e., at least eight glasses of water a day) and eating high-fiber foods (i.e., fruits, vegetables, and whole grains). For backaches, it is important to maintain good posture, avoid lifting very heavy objects, and wear low-heeled shoes. Eating smaller amounts of food more frequently and avoiding fried or spicy foods will help to relieve heartburn or indigestion. When the woman sits down to rest, her feet should be elevated to prevent swelling of the ankles. Pregnant women should try not to stand for long periods of time.

By the third trimester (27-40 weeks), many women begin to experience a range of common symptoms. As the baby grows larger and begins to press against internal organs, a woman may feel somewhat breathless at times. Lying on her left or right side, not on her back, and using pillows to lean on in a semipropped position can relieve this. Leaking of urine may occur with coughing or sneezing, and frequent urination begins again. As the pelvis widens and the joints become looser, discomfort may be felt in the pelvic joints. Some women feel as if their legs cannot support their weight. This is the body's way of preparing for birth. The joints are loosening so that the baby can fit through the pelvis. Stretch marks may develop on the abdomen, breasts, and thighs, and a dark line may appear from the navel to the pubic hair. A thin fluid called "colostrum" may be expressed from the nipples. Hemorrhoids may develop. Gums may become sensitive and bleed more easily, and eyes may dry out, making contact lenses uncomfortable to wear. Pica (a craving to eat substances other than food) may occur. Varicose veins may be a problem in the second half of pregnancy. This can be alleviated to a certain degree by wearing support hose, not standing for long periods of time, and resting with the feet up. Chloasma (a brown pigment) may appear on the face. This is due to the hormones of pregnancy and will disappear some time after delivery. Weak, irregular, painless tightenings of the uterus become more intense as the body practices for labor. These are called Braxton-Hicks contractions, and feel as if the baby is balling up. In most women, genuine labor consists of regular contractions that increase in intensity. Kicks from an active baby may cause sharp pains, and lower backaches are common. It is important for women in the third trimester to rest often and avoid straining themselves. When resting or sleeping, it may be more comfortable to lie on the left or right side with one leg bent, placing pillows under the stomach and between the knees. Weight gain will continue as it did in the second trimester.

In a woman's first pregnancy (later in repeat pregnancies), the baby's head drops down low into the pelvis by the last four weeks. This may relieve pressure on the upper abdomen and the lungs, allowing a woman to breathe more easily. This new position, however, does place more pressure on the bladder.

Total weight gain recommended in pregnancy is 25-35 pounds (12-16 kg) for women of normal weight for their height. Underweight women should possibly gain up to 40 pounds (18 kg), and overweight women should limit weight gain to 15-25 pounds (7-11 kg). Increased fluid volume makes up 2-3 pounds (0.9-1.4 kg); breast enlargement provides 1-2 pounds (0.45-0.9 kg); 2 pounds (0.9 kg) comes from enlargement of the uterus; and amniotic fluid is about 2 pounds (0.9 kg). At term, an infant weighs about 6-8 pounds (2.7-3.6 kg), and the placenta weights 1-2 pounds (0.45-0.9 kg). Usually 4-6 pounds (1.8-2.7 kg) are due to maternal stores of fat and protein that are important for breast-feeding.

While many of the symptoms mentioned are considered normal, there are others that may indicate the presence of complications. A pregnant woman experiencing any of the following should contact her doctor or midwife immediately:

  • abdominal pain
  • rupture of the amniotic sac or fluid leaking from the vagina
  • bleeding from the vagina
  • no fetal movement for 24 hours (after the sixth month)
  • continuous headaches
  • marked, sudden swelling of eyelids, hands, or face
  • dim or blurry vision
  • persistent heartburn (unrelieved by antacids ) or a burning sensation in the chest area
  • persistent vomiting
FDA categories for drugs during pregnancy
Source: U.S. Food and Drug Administration.
Category ANo risk to fetus in first trimester demonstrated in controlled studies, and no evidence of risk in other trimesters.
Category BNo fetal risk shown in animal studies but no controlled studies in prenant women are available, or animal studies showed an adverse effect not confirmed in controlled studies with pregnant women in the first trimester.
Category CAdverse effects on fetus found in animal studies but no controlled studies in women, or studies in women and animals are not available. Give drug only if benefit justifies the possible risk to the fetus.
Category DPositive evidence of fetal risk exists but the benefits may be acceptable despite the risk, as in life-threatening situations or serious disease.
Category XFetal abnormalities have been demonstrated or evidence for fetal risk exists, and the risk involved with using the drug outweighs any benefit.


Many women discover they are pregnant after a positive home pregnancy test. Urine tests check for the presence of human chorionic gonadotropin (hCG), which is produced by the placenta. The newest home tests can detect pregnancy six to nine days after a missed menstrual period—sometimes earlier. The manufacturers of these tests claim an accuracy rate of 96-99%; but some factors, such as taking medications, sunlight, heat, and medical conditions can affect the test. A negative result followed by no menstrual period within a week indicates the need to repeat the pregnancy test. While home tests are very accurate, they are less accurate than a pregnancy test performed in a lab. For this reason, women may want to consider having a second test at their doctor's or midwife's office to verify the accuracy of the result.

Blood tests to determine pregnancy are generally used when a very early diagnosis of pregnancy is needed. This more expensive test, which also looks for hCG, can produce a result within nine to 12 days after conception.

Approximately 3% of live-born infants have a major defect. There are tests that can be performed to determine many of these. As of 2001, there was a prenatal diagnostic screening test for determining the risk of neural tube defects, abdominal wall defects, Down syndrome, and trisomy 18. The triple-marker screening test measures levels of alpha fetoprotein (AFP), human chorionic gonadotropin (hCG), and unconjugated estriol. AFP is a protein produced in the fetal yolk sac during the first trimester and later by the fetal liver. Abnormally high levels of this protein are associated with severe neural tube defects. Human chorionic gonadotropin (hCG) is a hormone produced and secreted by the placenta. During early normal pregnancy, the level of this hormone rises rapidly, then begins to decline between the 10th and 20th week of gestation. High levels of hCG during the second trimester are associated with Down syndrome. Unconjugated estriol is produced by the placenta, the fetal adrenal glands, and the liver. It rises as normal pregnancy progresses, and its values are often lower with Down syndrome. A woman must have her blood sample drawn between the 15th and 20th week of gestation in order for this test to be accurate.

Other tests are recommended for women who are at higher risk for having a child with a birth defect. This includes women who have previously given birth to a child with a defect, or who have a family history of birth defects; women who have been exposed to certain drugs or high levels of radiation; and women 35 years of age or older. The presence of any of these risk factors warrants not only genetic counseling, but consideration of an ultrasound by a specialist, chorionic villi sampling (CVS), and/or amniocentesis.

First prenatal visit

During a woman's first prenatal visit, the following diagnostic tests are usually performed:

  • complete blood count (CBC), for anemia
  • blood type, Rh, and antibody screen
  • syphilis (VDRL)
  • rubella titer (German measles)
  • hepatitis B virus (HBV)
  • urinalysis and culture
  • Pap smear
  • cervical cultures for gonorrhea and chlamydia
  • recommendation of HIV antibody test, with counseling

A screening test for gestational diabetes is performed between 24 and 28 weeks' gestation by giving the woman a 50 g glucose drink, then drawing a blood sample one hour later to check the glucose level. A normal value is less than 130-140 mg/dL. A woman with a family history of diabetes, however, should be tested on her first visit to the obstetrician/gynecologist or nurse-midwife.


Women with heart disease, diabetes, lupus, and some hereditary conditions should consult a health professional before getting pregnant, as these conditions increase the risk of morbidity and mortality for both the mother and child.

Prenatal care is vitally important for the health of the unborn baby. During the first trimester, the woman should receive 0.4 to 0.8 mg (micrograms) of folic acid daily to reduce the chance of neural tube defects. Ideally, this daily dose of folic acid should begin at least one month prior to conception. Generally, requirements for all vitamins are increased during pregnancy. Prenatal vitamins prescribed by a physician or midwife usually contain the recommended amount of folic acid, and some contain a stool softener to offset the constipating effects of iron. Following delivery, vitamins are also recommended for the breast-feeding woman. Most pregnant women need at least 2,300 calories a day; these should come from good sources of protein, green leafy vegetables, fresh fruit, and breads and cereals. Small meals may be eaten frequently throughout the day.

Since most medications can pass from the mother to the baby, no medication (not even a nonprescription drug) should be taken except under medical supervision. No drug should be considered completely safe (especially during early pregnancy), although many physicians and nurse-midwives approve their patients' use of some drugs, including acetaminophen. Drugs taken during the first three months of a pregnancy may interfere with the normal formation of a baby's organs, leading to birth defects. Drugs taken later on in pregnancy may slow the baby's growth rate, or they may damage specific fetal tissue (such as developing teeth).

To increase the chance of having a healthy baby, a pregnant woman should avoid the following:

  • smoking
  • alcohol
  • street drugs
  • large amounts of caffeine (more than a cup or two of coffee per day)
  • artificial sweeteners (although clinicians debate this)
  • more than 5,000 U of vitamin A


Pregnancy is a natural condition and not a disease. If a woman takes good care of herself, plans her pregnancy with medical counseling, maintains optimal health, and obtains good prenatal care, the pregnancy and birth experience will be joyful events. In choosing a caregiver, the pregnant woman must consider what she wants for herself and her baby. The standard hospital experience, despite the homey decorations and presence of family members, frequently demands that the woman in labor remain in bed attached to a fetal monitor. Bed rest and hydration by IV slow down labor, even when normal. The ability to walk and change position during labor alleviates discomfort felt during contractions.

Many health care facilities are now utilizing nurse-midwives to attend births; their approach permits fewer interventions. Nurse-midwives are nurses who have received additional training in order to care for women having normal pregnancies and birth. If any complications should arise, they are well trained to detect them early, and they will call in the physician with whom they work. The pregnant woman and her partner should make a birth plan for their experience and present it to their caregiver early in the pregnancy to determine if the individual meets their expectations. The American College of Nurse-Midwives can provide the pregnant woman with a list of midwives in her area.


Alpha-fetoprotein— A substance produced by a fetus' liver that can be found in the amniotic fluid and in the mother's blood. Abnormally high levels of this substance suggests there may be defects in the fetal neural tube, a structure that will include the brain and spinal cord when completely developed. Abnormally low levels suggest the possibility of Down syndrome.

Alveolus/Alveoli— A little hollow space in the thin-walled chambers of the lungs which is surrounded by capillaries for the exchange of carbon dioxide and oxygen.

Amniotic fluid— The fluid or 'bag of waters' that the fetus floats in and maintains a constant body temperature. It is normally clear.

Anomaly— A marked deviation from normal.

Braxton-Hicks contractions— Short, fairly painless uterine contractions during pregnancy that may be mistaken for labor pains. They allow the uterus to grow and help circulate blood through the uterine blood vessels.

Cervix— The narrow lower end of the uterus.

Chloasma— A skin discoloration common during pregnancy, also known as the "mask of pregnancy." The blotches may appear on the forehead, cheeks, and nose, and may merge into one dark mask. Chloasma usually fades after pregnancy, but it may become permanent or recur with subsequent pregnancies.

Diffusion— The process of being widely spread.

Embryo— An unborn child during the first nine weeks of development following conception (fertilization with sperm). For the rest of pregnancy, the embryo is known as a fetus.

Fallopian tube— Either of two slender tubes, one on each side of the uterus, where fertilization takes place.

Fetus— An unborn child from the end of the ninth week after fertilization until birth.

Gestation— The period of development of an embryo and fetus or the duration of a pregnancy in a human.

Human chorionic gonadotropin (hCG)— A hormone produced by the placenta during pregnancy.

Osmosis— The diffusion of a substance across a membrane.

Placenta— The organ that develops in the uterus during pregnancy that links the blood supplies of the mother and baby.

Rhythm method— The oldest method of contraception, with a very high failure rate, in which partners periodically refrain from having sex during ovulation. Ovulation is predicted on the basis of a woman's previous menstrual cycle.

Uterus— The hollow muscular organ in females in which the fertilized egg becomes embedded and develops into an embryo and then a fetus.

Villus/Villi— A small protrusion or projection from the surface of a membrane.

Childbirth education classes for the woman and her partner are extremely important in helping a couple prepare for labor and delivery. Pregnant women should be made aware of the options for the location and circumstances for birth. Increasing numbers of women are having their babies in their own homes or in birthing centers, as opposed to a traditional hospital setting.


There are many methods of contraception available to prevent pregnancy. In order of least to most effective, these include:

  • spermicide alone
  • natural (rhythm) method
  • diaphragm or cervical cap
  • condom alone
  • diaphragm with spermicide
  • condom with spermicide
  • intrauterine device (IUD)
  • contraceptive pill
  • sterilization (either man or woman)
  • abstinence



Dahl, Gail. Pregnancy and Childbirth Tips. Canada: Dahl, Gail. 1999. (1-888-999-2080).

Goer, Henci. The Thinking Woman's Guide to a Better Birth. Perigee Books, 1999.

Kippley, Sheila Matgen. Breastfeeding and Natural Child Spacing: How Ecological Breast-feeding Spaces Babies. Cincinnati: The Couple to Couple League, 1999.

Moore, Keith L., and T. V. N. Persaud. The Developing Human. Philadelphia: W. B. Saunders Co., 1998.

Olds, Sally B., London, Marcia L., Ladewig, Patricia A. Wieland. Maternal-Newborn Nursing: A Family and Community-Based Approach. Upper Saddle River, NJ: Prentice Hall Health, 2000.

Starr, Winifred L., Shannon, Maureen T., Lommel, Lisa L., Gutierrez, Yolanda M. Ambulatory Obstetrics. San Francisco: UCSF Nursing Press, 1999.


The Couple to Couple League. P.O. Box 111184, Cincinnati, OH 45211-1184. (513) 471-2000. 〈〉.

Doulas of North America. 13513 North Grove Drive, Alpine, UT 84004. (801) 756-7331. 〈〉.

Healthy Mothers, Healthy Babies National Coalition. 121 North Washington St., Suite 300, Alexandria, VA 22314. (703) 836-6110. 〈〉.

National Institute of Child Health and Human Development. Bldg 31, Room 2A32, MSC 2425, 31 Center Drive, Bethesda, MD 20892-2425. 〈〉.


"The Art and Science of Birth." Midwifery Today. 〈〉.

Planned Parenthood. 〈〉.

"Pregnancy Information." 〈〉.

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