From Encyclopedia of Sex and Sexuality
Pregnancy is the period beginning at conception and lasting until the delivery, during which the fetus develops in the womb into a viable infant (although some pregnancies may end in miscarriage). Pregnancy is generally divided into three stages, or trimesters.
 Stages of Pregnancy
 The First Trimester
The first trimester is counted from the first day of the last menstrual period through the twelfth week of pregnancy. Approximately seven days after the egg is fertilized and implants itself in the uterus, it continues to divide and multiply to form the placenta and embryo. Most organs of the embryo are formed in the fourth to eighth week after conception. The heartbeat can ordinarily be detected by ultrasound in the eighth or ninth week; the limb buds appear and orbits of the eyes and the eyelids are formed then too. The spinal canal forms as a groove in the back, which closes by the fifth week. The genitalia are differentiated between the eleventh and fourteenth weeks. By the end of the twelfth week, the fetus has a human shape and, because all its organ systems are now formed, the living organism is referred to as a fetus rather than an embryo.
It is the differentiation of the cells of the body into organ systems during the first trimester that makes this time so critical. It is very important for the mother to maintain a healthy diet and avoid drugs such as tobacco and alcohol, which can be potentially harmful to the growing embryo. Tobacco use can result in low birth-weight infants, and alcohol abuse can result in fetal alcohol syndrome, in which the baby grows poorly in utero, and, after birth, has a wide variety of disorders including mental retardation, behavioral disabilities, and abnormal facial characteristics (See Fetal and Infant Substance Abuse Syndromes).
During the first trimester maternal symptoms such as fatigue, nausea, vomiting, and increased frequency of urination are common and represent normal adaptation to the body changes occurring as a result of the pregnancy. There is an enlargement and tenderness of the breasts, increased pigmentation, particularly about the breast and abdomen, and occasionally menstrual-like cramps, which are normal if not accompanied by bleeding. Most of these symptoms are resolved by the twelfth to fourteenth week of pregnancy and they generally impose no limitation on the mother’s physical activity.
Spontaneous abortion (or miscarriage) occurs in nearly 20 percent of all pregnancies. It is most often the result of fetal chromosomal abnormalities that are incompatible with life. More than 80 percent of miscarriages occur during the first twelve weeks of pregnancy and, when due to developmental or chromosomal abnormalities, no preventive treatment is warranted, useful, or beneficial. A miscarriage is more apt to occur in a first pregnancy.
 The Second Trimester
The second trimester continues from the thirteenth gestational week until the twenty-fourth or twenty-fifth week. This is a period of continuing development of the organ systems formed during the first trimester and is the period of the most rapid growth of the fetus. The fetus will increase its weight from 14 grams (the weight of a penny) to 820 grams (1 1/2 pounds) in a matter of three months. During this time the fetal bones become increasingly calcified and the nervous system and intestinal tract become more mature.
In the second trimester the mother will begin to feel fetal movement (quickening) at about twenty weeks of pregnancy. The occurrence of leg cramps and increasing maternal weight-gain and increase in abdominal girth occur, especially toward the end of this trimester. Other common symptoms include heartburn, back pain, and occasional lower abdominal cramping from stretching of the ligaments supporting the uterus. These symptoms are all normal and generally minimal. They serve only to remind the woman of the changes her body is undergoing.
 The Third Trimester
The third trimester is characterized by marked fetal growth and maturation. Fat is deposited under the skin and the pulmonary system begins to mature in preparation for its breathing functions ouside of the uterus. The fingernails and hair grow, eyelids develop, and the eyes open. By the thirty-seventh completed week of gestation the fetus is considered mature. If born any time after this period, it should survive and do well with no special care in the newborn nursery; fetuses born early in the third trimester of pregnancy now have good chances of survival with modern perinatal and neonatal therapy, but obviously the earlier in the third trimester a baby is delivered, the more medical care will be required.
The average pregnancy lasts 266 days from conception to delivery in humans. There is a remarkable consistency to this length of gestation. What initiates labor at the end of pregnancy still remains one of the true mysteries of life. The uterus begins contracting in a regular fashion and these contractions increase in frequency and intensity as labor progresses. Most often labor begins before the membranes, or “bag of waters,” rupture, but occasionally these membranes rupture prior to the onset of labor. When this loss of fluid occurs at term, spontaneous labor nearly always ensues within twenty-four hours. Passage of a bloody mucus plug may be a sign of impending labor, but it is an unreliable signal, particularly after the first pregnancy.
 Pregnancy Symtoms and Tests
Pregnancy is usually suspected when a sexually active woman has an abrupt cessation of menstruation. She may experience nausea and vomiting, an increased frequency of urination, fatigue, and often an increased size and tenderness of her breasts. However, these symptoms can be vague, and many pregnancies advance to eight weeks or more before suspicion increases and a pregnancy test is obtained. Fetal movement is a later symptom of pregnancy and can be detected by the woman as early as sixteen weeks after conception. Known as “quickening,” it is frequently described as a “flutter” in the abdomen.
Most home pregnancy tests performed on a urine specimen purport to detect a pregnancy within a week of the missed menstrual period; but these tests can have incorrect results reaching above 10 percent, and a diagnosis of pregnancy should be confirmed by pelvic examination or, if necessary, a serum pregnancy test. This sensitive blood test can identify the hormone produced by the fertilized egg and placenta in as little as six days after fertilization, the day the egg is implanted in the uterus.
Physical and pelvic examinations give other clues to the presence of pregnancy. An enlarged uterus, a darkening of the areolar part of the breast around the nipple, and increased pigmentation of the abdomen, as well as softening and a bluish hue of the cervix, are all suggestive of pregnancy.
Pregnancy is confirmed by the definition of a fetal heart beat or by visualization of the gestational sac containing a fetus during an ultrasound examination. The fetal heart beat can be heard as early as eight weeks after conception by a specialized microphone known as a Doppler, but in most women the heartbeat is detected between ten and fourteen weeks of gestation by Doppler and at twenty weeks with a fetoscope, a specialized stethoscope. A transvaginal ultrasound examination can be diagnostic as early as the middle of the fourth week of gestation: an intrauterine gestational sac can be seen and fetal cardiac activity can be detected by the end of the sixth week.