Pregnancy, or gestation, is the period during which one single cell develops into a fully formed human being within the mother’s body. It begins at the moment of conception and normally continues for approximately 266 days, or nine calendar months, until the child is born.
Stages of development
The embryo starts life as a single fertilized egg no larger than a pinpoint. Contained within its nucleus, however, are the hereditary or genetic units that determine the infant’s ultimate development. These are carried on twisted ribbonlike strands of DNA within the chromosomes, which provide the “instructions” for and control the manufacture of the proteins that will build up the new body.
Development begins with cleavage, or cell division. The initial single cell undergoes repeated mitotic division to form firstly a solid, many-celled morula, and subsequently a hollow sphere, the blastocyst, which on about day seven implants in the endometrium. Clustered to one side of the blastocyst is a mass of cells the embryo which continue to divide. This is a complex process of rearrangement whereby cells migrate into layers and take up approximately definitive positions inside the embryo. The embryo invaginates, or pushes in on itself, so that it at first resembles a diminutive cuplike structure and subsequently an elongated pouch. By about the 12th day after conception, two distinct layers of precursor cells an inner endoderm and outer ectoderm have formed. And during the following week a third middle, or mesoderm layer, is produced between them.
As the cells migrate they also differentiate, changing structure and function to be transformed into the specialized cells, tissues, or organ types of the mature body. As individual cells differentiate, so too the entire mass begins to develop a rudimentary human shape.
All body systems and vital organs are formed within the first three months, though nearly all the body systems still have much developing to do. From the endoderm develops first a primitive gut, then the respiratory tubes, lungs, liver, and digestive organs; from the ectoderm develop the skin, sense organs, and nervous system; the mesoderm is the source of all the body’s connective tissues, bone, cartilage, muscles, heart, blood vessels, and urogenital system.
Nourishment is obtained from the mother’s body via a complex life-support system. First to develop is the amniotic sac, a fluid-filled bag of membranes in which the developing embryo is cushioned and protected at constanttemperature. A cap-shaped pad of tissue the placenta forms at the point of implantation and to this the embryo is attached by the umbilical cord. Maternal and embryonic bloodstreams remain separate but nutrients and waste are exchanged from one to the other via the placenta.
By the end of the third month, although the fetus is only about 3.5 inches (9 centimeters) long, it is easily recognizable as human. Most of the major internal organs are fully formed, the circulatory system functions, and rudimentary genitals have appeared. From this point (or in some cases from as early as eight weeks), the embryo is called a fetus, which grows and matures during the remaining six months until birth.
The course of pregnancy
For most women the first sign of pregnancy is the absence of menstruation. Other early signs include nausea, increased urination, and some heaviness in the breasts. Pregnancy can, however, be confirmed by a urine test a few weeks after the missed menstrual period. This will detect chorionic gonadotropin, a hormone produced by the placenta.
Most symptoms of pregnancy are caused by changed hormonal levels in the body and, from the third month, by the increased pressure and size of the fetus. High levels of estrogen and progesterone are responsible for feelings of nausea and possibly for the emotional swings of the first months. After about six weeks, hormonal activity also prepares the breasts for nursing. Breasts may feel itchy or heavy as they begin to enlarge; veins become prominent and, from about the 12th week, a thin fluid—colostrum—is secreted. The areolae become mottled as pigmentation increases, and increased pigmentation may also appear on the face, lower abdomen, and genitals.
As the fetus increases in size, the abdomen swells and the mother’s weight gradually increases an average gain of 30 to 35 pounds (13 to 16 kilograms). Posture may alter to compensate, and many women experience backache and lethargy. Other uncomfortable side effects may include constipation, hemorrhoids, indigestion, and varicose veins.
The movements of the developing infant can usually be felt by about 20 weeks; by about the 32nd week, pressure may lessen as the baby engages, when the fetal head passes the pelvic inlet, having turned head down in the pelvis.
Ensuring health
Pregnancy is a natural condition, not an illness. Nevertheless, a good diet, exercise, regular prenatal care, and medical supervision are the best way to ensure a good pregnancy and healthy baby. A woman should visit her physician as early as possible for a thorough medical examination during which weight, blood pressure, blood group, and medical history can be checked. Thereafter, regular visits to a clinic or physician are essential to monitor progress. Prenatal medical care can help prevent or arrest complications, such as miscarriage or toxemia—a serious condition in which excessively elevated blood pressure is present, along with swelling of hands, face, and feet, and the appearance of protein in the urine.
A woman can do a great deal herself to ensure health. The baby obtains all its nourishment from the mother, so a balanced diet with adequate iron and calcium is essential. Smoking, excessive alcohol, and certain drugs cause damage and must be avoided. Drugs should only be taken under strict medical supervision.
Contact with infectious diseases, particularly rubella, during the first three months can also cause damage to the fetus. Some factors are uncontrollable, however, notably chromosomal abnormalities resulting in such genetic disorders as Down’s syndrome. These may cause early, spontaneous miscarriage but if not, some can be detected by amniocentesis the testing of the amniotic fluid at a relatively early stage (between the 12th and 16th weeks).
Birth
Birth, or parturition, marks the culmination of nine months of pregnancy. For the expectant mother it can be exciting and alarming, but anxiety can be lessened by an understanding of the processes involved.
During the final weeks of pregnancy, the woman’s body prepares itself for labor. The cervix softens and uterine contractions that have been occurring throughout pregnancy become more noticeable. Finally, the ligaments of the pelvis soften and become more flexible, the cervix begins to be “effaced” or thinned and the baby’s head “engages” or drops low in the pelvis.
Stages of labor
Labor is divided into three stages. First, the cervix dilates so that the baby can pass through; second, the baby is delivered; and third, the placenta and membranes are expelled. These stages are not clear-cut but tend to merge into each other. No one knows for sure what actually initiates labor, although it is thought that hormones produced by the mature fetus may stimulate the production of prostaglandins, which in turn act on the uterus. Various signs indicate that labor is imminent. Regular contractions occurring every 3 or 5 minutes are the most common sign; others include the expulsion of a mucous plug from the cervix—called the “show”—and a gush of fluid as the amniotic sac ruptures.
The first stage is characterized by increasingly intense muscular contractions as muscle fibers pull the lower uterus and cervix up and around the head of the fetus. This stretching and pulling in turn dilates the cervix to allow the baby to pass through. The process may take anything up to 20 hours; usually, however, it lasts about 8 to 18 hours for a woman having her first child, and about half this for subsequent children.
As dilation proceeds, contractions become more frequent and intense until toward the end of the first stage the transition period when they occur every two to three minutes, each contraction lasting about 60 seconds. This period is particularly painful and a woman may also experience nausea, backache, and leg cramps. Increasing too may be an uncontrollable desire to push or bear down as the pressure from the baby’s head intensifies within the pelvis.
Once the cervix is fully dilated, the second stage is entered and the powerful “bearing down” reflex is accentuated. From this point, the woman can participate actively, bearing down with each contraction to push the baby out of her body. In a normal delivery the baby is born head first. The head rotates beneath the pubic arch and, as it emerges, rotates back to its original position. Shoulders and trunk follow in the same way and the baby is born. The baby breathes in and makes a first cry. The umbilical cord is usually cut at this stage.
The third and final stage occurs within the next 15 minutes as the placenta and umbilical cord are expelled. This is a painless process that may be aided by light pulling on the cord. Once delivered, the placenta is checked to ensure that nothing has been left in the uterus that might cause hemorrhage or infection. Subsequent contraction of the uterine wall usually stops further bleeding.
Relief of pain
Childbirth is painful and the damaging effects on both mother and child of a prolonged and distressing birth are now well known. Today the emphasis is on “natural” drug-free labor, and the woman who understands what is happening and who is adequately prepared with breathing and other exercises can frequently give birth unaided and with a minimum of pain and discomfort. It is also essential that a woman in labor is not left alone; the sympathetic presence of her partner or another relative is actively encouraged. Many women do, however, choose some form of pain relief. Among drugs available are: the epidural anesthetics, which are administered around the spine and block out all sensation in the lower part of the body and analgesics such as pethidine. These drugs can affect the baby, however, and today they are used with considerable caution.
Problem births
Most births are perfectly normal but problems can and do occur. Breech presentation, where the baby is born feet or buttocks first, is one example. In this case, duration of labor is critical: too fast a labor may result in damage; too long a delivery may cause oxygen deprivation. In a breech presentation, delivery is usually in three stages: breech and legs, shoulders, and then head, forceps (surgical tongs) being used to ease the head out. Where a normal vaginal delivery is impossible, a baby may be delivered by Caesarean section. This involves making an incision in the mother’s lower abdomen and delivering the baby through it.
Labor may sometimes be induced artificially when the health of mother or baby is at risk. Reasons for induction include preeclampsia, postmaturity of the fetus, Rh factor incompatibility, or diabetes. Induction may involve artificially rupturing the amniotic sac or the intravenous infusion of oxytocin or pitocin to stimulate contractions. Finally, where there is a danger of the baby’s head tearing the perineum, an incision in the vagina an episiotomy-may be made.
After the birth
The body returns to normal remarkably quickly. Initially there is a steady loss of a bloody substance lochia from the vagina as the placental site and uterine lining break down. This should end after 4 to 6 weeks, and within six weeks uterus, cervix, and vagina should have returned to normal. Postnatal exercise helps to tone up stretched muscles. Breasts secrete colostrum for the first two or three days after birth, before the milk flow begins.
One common feature of the postbirth period is depression, which may be caused by hormonal imbalance or emotional and social factors. Whatever the cause, however, postnatal depression is distressing, and sympathetic treatment and support from family and friends are essential for a speedy recovery. It should not be confused with the much rarer postpartum depression, which is a far more severe form of mental disturbance.