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Labor may start too early (before the 37th week of pregnancy) or may start late (after the 41st to 42nd week of pregnancy). As a result, the health or life of the fetus may be endangered. Labor may start too early or late when the woman or fetus has a medical problem or the fetus is in an abnormal position.
No more than 10% of women deliver on their specified due date (usually estimated to be about 40 weeks of pregnancy). About 50% of women deliver within 1 week (before or after), and almost 90% deliver within 2 weeks of the due date. Determining the length of pregnancy can be difficult, because the precise date of conception often cannot be determined. Early in pregnancy, an ultrasound examination, which is safe and painless, can help determine the length of pregnancy. In mid to late pregnancy, ultrasound examinations are less reliable in determining length of pregnancy.
Premature
Rupture of the Membranes:
In about 10% of normal pregnancies, the fluid-filled membranes containing the fetus rupture before labor begins. Contractions usually begin within 12 to 48 hours. Rupture of the membranes is commonly described as "the water breaks." The fluid within the membranes (amniotic fluid) then flows out from the vagina. The flow varies from a trickle to a gush. As soon as the membranes have ruptured, a woman should contact her doctor or midwife.
If labor does not begin within 24 to 48 hours, the risk of infection of the uterus and fetus increases. Therefore, a doctor or certified midwife usually artificially starts (induces) labor, depending on whether or not the fetus is mature enough for delivery. The doctor may analyze the amniotic fluid to determine if the fetus's lungs are mature enough. If they are, labor is induced and the baby is delivered. If they are not, the doctor usually does not induce labor.
The woman's temperature and pulse rate are usually recorded at least twice daily. An increase in temperature or pulse rate may be an early sign of infection. If an infection develops, labor is promptly induced and the baby is delivered. Very rarely, if the amniotic fluid stops leaking and contractions stop, the woman may be able to go home. In such cases, the woman should be seen by her doctor at least once a week.
Preterm
Labor:
Because babies born prematurely can have significant health problems (see Problems in Newborns: Prematurity), doctors try to prevent or stop labor that begins before the 34th week of pregnancy. What causes preterm labor is not well understood. However, a healthy lifestyle and regular visits to the doctor or midwife during pregnancy are helpful. Preterm labor is difficult to stop. If vaginal bleeding occurs or the membranes rupture, allowing labor to continue is often best. If vaginal bleeding does not occur and the membranes are not leaking amniotic fluid, the woman is advised to rest and to limit her activities as much as possible, preferably to sedentary ones. She is given fluids and may be given drugs that can slow labor. These measures can often delay labor for a brief time.
Drugs that can slow labor include magnesium sulfate and terbutaline. Magnesium sulfate given intravenously stops preterm labor in many women. However, if the dose is too high, it may slow the woman's heart and breathing rates. Terbutaline given by injection under the skin also can be used to stop preterm labor. However, as a side effect, it increases the heart rate in the woman, fetus, or both. Sometimes ritodrine is used instead of terbutaline.
If the cervix opens (dilates) beyond 2 inches (5 centimeters), labor usually continues until the baby is born. If doctors think that premature delivery is inevitable, a woman may be given a corticosteroid such as betamethasone. The corticosteroid helps the fetus's lungs and other organs mature more quickly and reduces the risk that after birth, the baby will have difficulty breathing (neonatal respiratory distress syndrome).
Postterm Pregnancy
and Postmaturity:
In most pregnancies that go a little beyond 41 to 42 weeks, no problems develop. However, problems may develop if the placenta cannot continue to maintain a healthy environment for the fetus. This condition is called postmaturity.
Typically, tests are started at 41 weeks to evaluate the fetus's movement and heart rate and the amount of amniotic fluid, which decreases markedly in postmature pregnancies. The fetus's rate of breathing and heart sounds may also be monitored. Doctors can check on the fetus's well-being with electronic fetal heart monitoring (see Monitoring the Fetus ). Typically, at 42 weeks, labor is induced, or the baby is delivered by cesarean section.
Labor
That Progresses Too Slowly:
If labor is progressing too slowly, the fetus may be too big to move through the birth canal (pelvis and vagina). Delivery by forceps, a vacuum extractor, or cesarean section may be necessary. If the birth canal is big enough for the fetus but labor is not progressing, the woman is given oxytocin intravenously to stimulate the uterus to contract more forcefully. If oxytocin is unsuccessful, a cesarean section is performed. If the baby is already in position to be delivered, forceps or a vacuum extractor may be used instead.
Last full review/revision February 2003
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