Pregnancy is the period from conception to birth. After the egg is fertilizedby a sperm and then implanted in the lining of the uterus, it develops intothe placenta and embryo, and later into a fetus. Pregnancy usually lasts 40 weeks, beginning from the first day of the woman's last menstrual period, andis divided into three trimesters, each lasting three months.

At the end of the first month, the embryo is about a third of an inch long, and its head and trunk--plus the beginnings of arms and legs--have started todevelop. The embryo gets nutrients and eliminates waste through the umbilicalcord and placenta. By the end of the first month, the liver and digestive system begin to develop, and the heart starts to beat.

In the second month, the heart starts to pump and the nervous system (including the brain and spinal cord) begins to develop. The 1 in (2.5 cm) long fetushas a complete cartilage skeleton, which is replaced by bone cells by month's end. Arms, legs and all of the major organs begin to appear. Facial features also begin to form.

By the third month the fetus has grown to 4 in (10 cm) and weighs a little more than an ounce (28 g). Now the major blood vessels and the roof of the mouth are almost completed, as the face starts to take on a more recognizably human appearance. Fingers and toes appear. All the major organs are now beginning to form; the kidneys are now functional and the four chambers of the heartare complete.

In the fourth month the fetus begins to kick and swallow, although most womenstill can't feel it move. Now 4 oz (112 g), the fetus can hear and urinate,and has established sleep-wake cycles. All organs are now fully formed, although they will continue to grow for the next five months. The fetus has skin,eyebrows and hair.

By the fifth month the fetus weighs up to a 1 lb (454 g) and measures 8-12 in(20-30 cm). The fetus experiences rapid growth as its internal organs continue to grow. At this point, the mother may feel the fetus move, and she can hear the heartbeat with a stethoscope.

Even though its lungs are not fully developed, a fetus born during the sixthmonth can survive with intensive care. Weighing 1-1.5 lbs (454-681 g), the fetus is red, wrinkly, and covered with fine hair all over its body. The fetuswill grow very fast during this month as its organs continue to develop.

There is a better chance that a fetus born during the seventh month will survive. The fetus continues to grow rapidly, and may weigh as much as 3 lbs (1.3kg). Now the fetus can suck its thumb and look around its watery womb with open eyes.

Growth continues during the eighth month but slows down as the baby begins totake up most of the room inside the uterus. Now weighing between 4-5 lbs (1.8-2.3 kg) and measuring 16-18 in (40-45 cm) long, the fetus may at this timeprepare for delivery next month by moving into the head-down position.

Adding 0.5 lb (227 g) a week during the ninth month as the due date approaches, the fetus drops lower into the mother's abdomen and prepares for the onsetof labor, which may begin any time between the 37th and 42nd week of gestation. Most healthy babies will weigh 6-9 lbs (2.7-4 kg) at birth, and will be about 20 inches long.

The first sign of pregnancy is usually a missed menstrual period, although some women bleed in the beginning. A woman's breasts swell and may become tender as the mammary glands prepare for eventual breastfeeding. Nipples begin toenlarge and the veins over the surface of the breasts become more noticeable.Nausea and vomiting are very common symptoms and are usually worse in the morning. Many women also feel extremely tired during the early weeks. Frequenturination is common, and there may be a creamy white discharge from the vagina. Some women crave certain foods, and an extreme sensitivity to smell may worsen the nausea. Weight begins to increase.

In the second trimester (13-28 weeks) a woman begins to look noticeably pregnant and the enlarged uterus is easy to feel. The nipples get bigger and darker, skin may darken, and some women may feel flushed and warm. Appetite may increase. By the 22nd week, most women have felt the fetus move. During the second trimester, nausea and vomiting often fade away, and the pregnant woman often feels much better and more energetic. Heart rate increases as does the volume of blood in the body.

By the third trimester (29-40 weeks), many women begin to experience a rangeof common symptoms. Stretch marks may develop on abdomen, breasts and thighs,and a dark line may appear from the navel to pubic hair. A thin fluid may beexcreted from the nipples. Many women feel hot, sweat easily and often findit hard to get comfortable. Kicks from an active fetus may cause sharp pains,and lower backaches are common. More rest is needed as the woman copes withthe added stress of extra weight. Braxton Hicks contractions may get stronger.

At about the 36th week in a first pregnancy (later in repeat pregnancies), the baby's head drops down low into the pelvis. This may relieve pressure on the upper abdomen and the lungs, allowing a woman to breathe more easily. However, the new position places more pressure on the bladder.

The average woman gains 28 lbs (12.7 kg) during pregnancy, 70% of it during the last 20 weeks. An average, healthy full-term baby at birth weighs 7.5 lbs(3.4 kg), and the placenta and fluid together weigh another 3 lbs (1.3 kg). The remaining weight that a woman gains during pregnancy is mostly due to water retention and fat stores.

In addition to the typical, common symptoms of pregnancy, some women experience other problems that may be annoying but which usually disappear after delivery. Constipation may develop as a result of food passing more slowly through the intestine. Hemorrhoids and heartburn are fairly common during late pregnancy. Gums may become more sensitive and bleed more easily; eyes may dry out, making contact lenses feel painful. Pica (a craving to eat substances otherthan food) may occur. Swollen ankles and varicose veins may be a problem inthe second half of pregnancy, and chloasma may appear on the face.

While the above symptoms are all considered to be normal, there are some symptoms that could be a sign of a more dangerous underlying problem. A pregnantwoman with any of the following signs should contact her doctor immediately:abdominal pain, rupture of the amniotic sac or leaking of fluid from the vagina, bleeding from the vagina, no fetal movement for 24 hours (after the fifthmonth), continuous headaches, marked, sudden swelling of eyelids, hands or face during the last three months, dim or blurry vision during last 3 months,or persistent vomiting.

Many women first discover they are pregnant after a positive home pregnancy test. Pregnancy urine tests check for the presence of human chorionic gonadotropin (hCG), which is produced by a placenta. The newest home tests can detectpregnancy on the day of the missed menstrual period.

Home pregnancy tests are more than 97% accurate if the result is positive, and about 80% accurate if the result is negative. If the result is negative andthere is no menstrual period within another week, the pregnancy test shouldbe repeated. While home pregnancy tests are very accurate, they are less accurate than a pregnancy test conducted at a lab. For this reason, women may want to consider having a second pregnancy test conducted at their doctor's office to be sure of the accuracy of the result.

Blood tests to determine pregnancy are usually used only when a very early diagnosis of pregnancy is needed. This more expensive test, which also looks for hCG, can produce a result within 9-12 days after conception.

Once pregnancy has been confirmed, there are a range of screening tests thatcan be done to screen for birth defects, which affect about 3% of unborn children. Two tests are recommended for all pregnant women: alpha-fetoprotein (AFP) and the triple marker test.

Other tests are recommended for women at higher risk for having a child witha birth defect. This would include women over age 35, who had another child or a close relative with a birth defect, or who have been exposed to certain drugs or high levels of radiation. Women with any of these risk factors may want to consider amniocentesis, chorionic villus sampling (CVS) or ultrasound.

There are a range of other prenatal tests that are routinely performed, including: pap test, gestational diabetes screening test at 24-28 weeks, tests forsexually transmitted diseases, urinalysis, blood tests for anemia or blood type, and screening for immunity to various diseases, such as German measles.

Prenatal care is vitally important for the health of the unborn baby. A pregnant woman should be sure to eat a balanced, nutritious diet of frequent, small meals. Many doctors prescribe pregnancy vitamins, including folic acid andiron supplementation during pregnancy.

No medication (not even a nonprescription drug) should be taken except undermedical supervision, since it could pass from the mother through the placentato the developing baby. Some drugs have been proven harmful to a fetus, butno drug should be considered completely safe (especially during early pregnancy). Drugs taken during the first three months of a pregnancy may interfere with the normal formation of the 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 the developing teeth).

To have the best chance of having a healthy baby, a pregnant woman should avoid: smoking, alcohol, street drugs, large amounts of caffeine, and artificialsweeteners. Childbirth education classes for the woman and her partner helpprepare the couple for labor and delivery.

There are many ways to avoid pregnancy. A woman has a choice of many methodsof contraception which will prevent pregnancy, including (in order of least to most effective): spermicide alone, natural (rhythm) method, diaphragm or cap alone, condom alone, diaphragm with spermicide, condom with spermicide, intrauterine device (IUD), contraceptive pill, sterilization (either a man or woman), and avoiding intercourse.

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