Birth is the process by which a fully developed fetus is expelled from the mother's uterus by the force of strong, rhythmic muscle contractions.
Usually lasting about 16 hours—but in some cases ranging from less than one hour to 48 hours—birth is the culmination of a series of complex physical processes. The length of time between fertilization and birth is called the gestation period. In humans, the gestation period is approximately nine months or 38 weeks.
Birth typically begins at the end of the gestation period when the mother's uterus begins to contract rhythmically, a process called labor. The initiation of labor leading up to birth is the result of a number of hormones,
Labor culminating in birth in humans begins with rhythmic contractions of the uterus that dilate the cervix. The cervix is normally tightly closed, and is sealed with a plug of mucus during gestation to protect the fetus from invading microorganisms. During the first stages of labor, the contractions of the uterus dilate the cervix, which widens to about 4 in (10 cm) to accommodate the passage of the fetal head.
In the last weeks of pregnancy, before labor begins, the uterus undergoes irregular contractions, which serve to exercise the muscles of the uterus and may even dilate the cervix. It is not unusual for a woman to go into active labor with a cervix that is already one or two centimeters dilated. During the last weeks of pregnancy, the cervix also thins out (or effaces), which makes dilation easier.
In preparation for birth, the fetus moves further down into the mother's pelvis. When labor begins, the fetus is usually positioned with its head engaged with the top of the cervix. This engagement is called "lightening" or "dropping." When labor begins, the contractions loosen the mucus plug in the cervix, which causes small capillaries in the cervix to break, and the mucus and blood are discharged from the vagina. This discharge is sometimes called "bloody show" and signals the onset of labor.
Another sign that may signal the beginning of labor is the rupturing of the amniotic sac. In the uterus the fetus is encased in a membrane (the amniotic sac) and literally floats in amniotic fluid. When uterine contractions begin, this sac ruptures and the amniotic fluid can leak from the uterus. Not all women experience an abrupt rupturing of the amniotic sac; in some the amniotic fluid gradually leaks out as labor progresses. Once the amniotic sac has ruptured, or the amniotic fluid begins to leak, labor usually progresses more rapidly.
During the first stage of labor, the cervix dilates about 0.5-0.6 in (1.2-1.5 cm) an hour. The uterine contractions are about 5-30 minutes apart, and last for 15-40 seconds. The end of the first stage of labor is associated with the strongest uterine contractions. Contractions are two to five minutes apart, and last for 45-60 seconds. The cervix opens rapidly at this point. This period of labor, sometimes called transition, is usually the most difficult for the mother. The contractions are very strong and close together, and nausea and vomiting are common. After the cervix has dilated to its full width of 4 in (10 cm), the contractions slow down somewhat to about three to five minutes apart. The fetus is then ready to be born, and the second stage of labor begins.
During the second stage, lasting about one to two hours, the mother uses her abdominal muscles to push the fetus through and out of the birth canal. The pushing is actually a reflex action, but if a woman can help the reflex by actively using her muscles, birth goes much faster. As the fetus moves down the birth canal to the vaginal opening, the head begins to appear. The appearance of the head at the opening of the vagina is called crowning. After the head is delivered, first one shoulder is delivered, then the other. The rest of the body follows.
After the baby is born, the umbilical cord that connects the fetus to the placenta is clamped. The clamping cuts off the circulation of the cord, which eventually stops pulsing due to the interruption of its blood supply. The baby now must breathe air through its own lungs.
Before delivery, the placenta separates from the wall of the uterus. Because the placenta contains many blood vessels, its separation from the wall of the uterus causes bleeding. This bleeding, if not excessive, is normal. After the placenta separates from the uterine wall, it moves into the birth canal and is expelled from the vagina. The uterus continues to contract even after the placenta is delivered, and it is thought that these contractions serve to control bleeding.
Today, women have many options for labor and birth. Some women deliver in a hospital with doctors and nurses close by to supervise the birth process. Others choose a nurse-midwife, a person who has been trained to deliver babies but who is not a physician. Still others choose home birth, attended either by a doctor or midwife, or sometimes both. Whatever option a woman chooses, it is important to get good medical care throughout the pregnancy. Periodic prenatal checkups are one of the best ways to avoid birth complications.
Many childbirth experts believe that the more knowledgeable a mother is about the birth process, the less fear and apprehension she will feel giving birth. Many childbirth classes prepare both mother and father for the birth experience and teach relaxation and breathing
Regional anesthesia is commonly used during labor and birth. In regional anesthesia, drugs are injected to deaden sensation around the spinal nerves that carry sensations from the pelvic region. Controversy about whether these drugs affect the fetus is ongoing, although some kinds of regional anesthesia affect the fetus less than others. General anesthesia, in which the mother is given drugs that put her to sleep, is rarely used today.
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