Pregnancy, also called gestation, is the period during which a female carries a developing baby within her body. The females of almost all species of mammals have a period of pregnancy before giving birth. In humans, pregnancy normally lasts about nine months.
Pregnancy begins when an egg (female sex cell) and a sperm (male sex cell) unite in a woman’s body. This union, called conception, forms a single cell called a fertilized egg. The fertilized egg attaches itself to the lining inside the woman’s uterus, a hollow, muscular organ in the abdomen. During the next nine months, the egg develops within the uterus into a fully formed baby. From about the third week to the eighth week after conception, all the baby’s major organs begin to form. During this time, the baby is called an embryo. From the ninth week until birth, the baby’s body grows in size while its organs mature. During this period, the baby is called a fetus. Pregnancy ends when the baby passes out of the mother’s body at birth.
This article discusses pregnancy and the experiences of the expectant mother up to the baby’s birth. For detailed information on the development of the baby during pregnancy and on the birth process, see Baby; Reproduction, Human; and Childbirth.
Becoming pregnant
A woman usually becomes pregnant by means of sexual intercourse. In sexual intercourse, a man’s penis discharges semen–fluid containing sperm–in the woman’s vagina. Pregnancy begins if a sperm penetrates an egg.
In addition, scientists have developed techniques that enable women to become pregnant without sexual intercourse. For example, in a process called in vitro fertilization, sperm are collected from a man and used to fertilize eggs in a laboratory dish. The eggs are then inserted into the woman’s uterus.
Planning for pregnancy.
Medical experts recommend that women who plan to become pregnant seek medical advice before conception, as much as a year before pregnancy. A woman should choose a health care provider skilled in giving such advice and in providing medical care during pregnancy. This person may be either a physician who is qualified to care for pregnant women or a nurse-midwife. In the United States, a nurse-midwife is a registered nurse specially trained and certified to care for pregnant women.
During the initial pregnancy visit, the woman provides information about her present health, past illnesses and pregnancies, and her family medical history. The health care provider gives the woman a thorough physical examination and orders certain medical tests. For example, blood and urine tests check for diseases or conditions in the mother that can harm a developing baby, including anemia, hepatitis B, syphilis and other sexually transmitted diseases, and rubella (German measles). One important blood test determines if the mother and baby have the same Rh factor, a substance on red blood cells. If they have different Rh factors, the baby may develop a serious blood disorder. See Rh factor.
The doctor or midwife counsels the woman about risks to the pregnancy based on her health, habits, medical history, or age. Women who have certain disorders in their family histories or who are over 35 may be advised to seek additional counseling to determine the chances of bearing a child with genetic defects.
Confirming pregnancy.
For the majority of women, the first sign of pregnancy is a missed menstrual period. A menstrual period is the time when a woman menstruates (discharges blood and tissue through the vagina). It occurs about every 28 days in most women of childbearing age. Each month, the lining of the uterus thickens in preparation for pregnancy. If the woman does not become pregnant, the lining breaks down and menstruation occurs. If the woman becomes pregnant, however, the thickened lining remains intact, helping to sustain the developing baby. As a result, menstruation stops after conception and resumes after the baby is born.
A woman who suspects she may be pregnant should have a pregnancy test as soon as possible. Pregnancy tests check for a hormone called human chorionic gonadotropin (hCG) in the woman’s blood or urine. This hormone is first produced by the fertilized egg and later by the placenta, a disk-shaped organ that forms in a pregnant woman’s uterus. The placenta enables the baby to obtain food and oxygen from the mother’s blood.
Blood tests performed in a laboratory can verify pregnancy as early as about seven days after conception. Women can test their urine for hCG using a home pregnancy test kit. Such tests are accurate beginning about the time of the woman’s first missed menstrual period. However, a qualified health care provider should confirm the results of a home pregnancy test.
Estimating the date of birth.
The health care provider uses the date of the expectant mother’s last menstrual period to estimate when the baby will be born. The birth typically is calculated to occur 280 days, or 40 weeks, from the first day of the woman’s last menstrual period. Throughout pregnancy, the estimated birth date may be adjusted somewhat based on the size of the uterus and other factors.
How pregnancy affects women
During pregnancy, a woman’s body undergoes enormous changes. Many changes result from increased levels of the hormones estrogen and progesterone in the woman’s blood. During pregnancy, these hormones are secreted into the bloodstream primarily by the placenta. The placenta also produces certain other hormones that trigger changes in a woman’s body. To help describe when changes occur in the woman and her developing baby, pregnancy is often divided into three three-month periods called trimesters.
Weight gain.
Women normally gain between 25 and 35 pounds (11 and 16 kilograms) during pregnancy. Several factors affect the amount gained, including a woman’s weight at the start of pregnancy and the number of babies she is carrying. At birth, the baby accounts for about 61/2 to 8 pounds (3 to 3.5 kilograms) of the weight gained and the placenta about 1 to 11/2 pounds (0.5 to 0.7 kilogram). The rest of the weight is mainly fat, blood and other fluids, and extra tissue in the breasts and uterus.
Changes in breasts.
A woman’s breasts change in certain ways throughout pregnancy so that they can produce milk to feed the baby after birth. For example, the glands in the breasts that make the milk and the ducts (tubes) that carry the milk to the nipples both enlarge. The breasts themselves become larger and more tender. The lactation (milk-making) process begins soon after the baby is born.
Changes in the uterus
prepare a woman’s body for the birth process, also called labor. During labor, contractions of the uterus and abdomen push the baby out of the woman’s body through the vagina.
The uterus expands to about 24 times its normal size during pregnancy, mostly because the muscle cells in the wall of the organ increase in size. Around the sixth month of pregnancy, the uterus begins to have irregular contractions that may be mild or intense.
Other common symptoms of pregnancy.
During the first trimester, hormonal changes cause many women to experience morning sickness (nausea and vomiting). By about the fourth month, however, morning sickness usually lessens or disappears.
Many pregnant women experience increased fatigue and the need to urinate frequently. Hormonal changes, weight gain, and other changes in the woman’s body contribute to fatigue. Frequent urination results mainly from the expanding uterus pressing on the bladder.
Other symptoms caused or made worse by the uterus pressing on organs include back pain, shortness of breath, discomfort in the pelvic region, heartburn, and constipation. In addition, varicose veins (enlarged veins in the legs) and hemorrhoids (enlarged veins around the anus) may occur.
Prenatal care
A pregnant woman’s health and habits can affect the growth and development of her baby. The term prenatal care refers both to medical care during pregnancy and to an expectant mother’s healthful lifestyle. Good prenatal care helps protect the health of the unborn baby.
Medical care.
A woman who becomes pregnant should receive periodic physical examinations and tests.
Routine examinations.
During each physical examination, the health care provider checks the mother’s weight, blood pressure, heart rate, and general physical condition. He or she listens to the baby’s heartbeat and measures and feels the mother’s abdomen to determine the growth and position of the fetus. Routine checkups also include a urine test to check for preeclampsia, a serious condition that can occur later in pregnancy, and gestational diabetes, a disorder that impairs the body’s ability to use sugar normally.
Special tests.
The health care provider may order one or more special tests. One of the most common of these is an ultrasound examination. In this examination, special equipment produces ultrasonic waves that reflect off the fetus. The returning echoes produce an image of the fetus on a screen. Ultrasound testing can help confirm the position and growth of the fetus and determine the number of babies the mother is carrying.
Special tests also may be given to determine whether the fetus has certain genetic defects, such as Down syndrome. In a test called amniocentesis, a physician takes a sample of the amniotic fluid, the watery liquid in the protective sac that forms around the baby. The fluid contains cells shed by the baby that can be tested for genetic disorders. In chorionic villus sampling, a sample of the placental tissue is analyzed for abnormalities. See Amniocentesis; Genetic counseling.
Lifestyle during pregnancy.
In general, a pregnant woman should eat a well-balanced diet, get plenty of sleep, and exercise regularly at a mild or moderate level. Doctors also recommend taking extra amounts of certain vitamins and minerals. Expectant mothers should not eat undercooked meat. Such meat may contain parasites that cause toxoplasmosis, a disease that can severely damage a developing baby. A cat’s body waste can also spread toxoplasmosis. As a result, pregnant women should not touch dirty cat litter.
Any chemical in the expectant mother’s blood can enter the developing baby’s bloodstream through the placenta. Some chemicals damage the developing baby or interfere with its growth. For this reason, pregnant women should not take medications, including over-the-counter drugs such as aspirin and nasal spray, unless advised to by a health care provider. Nonmedicinal drugs, alcoholic beverages, and cigarette smoke should also be avoided.
Pregnant women should avoid X rays and exposure to hazardous chemicals. They should also avoid excessive heat, such as in hot tubs or saunas. The body’s effort to keep cool in such heat can result in blood–and therefore oxygen–being drawn away from the uterus.
A pregnant woman can help herself, as well as her baby, by learning about labor and delivery. Childbirth classes provide a realistic preview of the birth process and teach natural pain-control techniques, such as breathing and relaxation. These methods can reduce or eliminate the need for pain medication during labor.
Complications
Common complications of pregnancy include (1) miscarriage, (2) ectopic pregnancy, (3) preeclampsia, and (4) premature birth.
Miscarriage,
also called spontaneous abortion, is the unintentional early ending of pregnancy by a natural cause before the 20th week of pregnancy. In a miscarriage, the embryo or fetus is expelled from the body by contractions of the uterus. More than 20 percent of pregnancies end in miscarriage. The majority of miscarriages occur during the first trimester.
Abnormalities in the embryo or fetus cause most miscarriages. Physical problems in the expectant mother also cause many miscarriages. Such problems, which usually cause more miscarriages later in the pregnancy than do fetal abnormalities, include infections; disorders of the immune system, cervix, or uterus; and diabetes or other diseases. Medical care before and during pregnancy can prevent some miscarriages.
An ectopic pregnancy
occurs when a fertilized egg implants itself outside the uterus, usually inside the fallopian tube, the tube through which the egg passes on its way from the ovary to the uterus. The developing baby cannot survive in an ectopic pregnancy. The pregnancy may be fatal to the mother if untreated.
formerly called toxemia of pregnancy, is one of the most serious conditions that can occur during the later months of pregnancy. Its cause is unknown. Physical symptoms include high blood pressure, headaches, sudden and excessive weight gain, and swelling, especially of the face, feet, and hands. If untreated, preeclampsia can lead to eclampsia, a condition that can result in the death of the mother, the baby, or both.
Premature birth.
Some infants are born prematurely–that is, before the 37th week. The more premature a baby is, the less it weighs and the higher its risk of death. A pregnant woman can lessen the risk of some premature births by good prenatal care.
