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 the length of pregnancy.
Premature Rupture of the Membranes
Premature rupture of the membranes is the leaking of amniotic fluid from around the fetus before labor starts.
Usually, the fluid-filled membranes containing the fetus rupture during labor. But in about 10% of normal pregnancies, the membranes rupture before labor starts. The membranes may rupture near the due date (at 37 weeks or later, which is considered full term) or earlier (called preterm rupture at less than 37 weeks). If rupture is preterm, delivery is also likely to be too early (preterm). Regardless of when premature rupture occurs, it increases the risk of infection of the uterus and the fetus. The fetus is also more likely to be in an abnormal position, and the placenta is more likely to detach too soon (placental abruption).
After the membranes rupture, contractions usually begin within 12 to 48 hours when the woman is near term but can take 4 days or longer if rupture occurs before 34 weeks of pregnancy. 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.
Using a speculum, the doctor or midwife examines the pelvis to confirm that the membranes have ruptured and to estimate how far the cervix (the lower part of the uterus) has opened (dilated).
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 the fetus is mature enough for delivery. To determine whether the fetus's lungs are mature enough, the doctor may take a sample of amniotic fluid, usually from the vagina, and analyze it. If the lungs are mature enough, labor is induced and the baby is delivered. If they are not, the doctor usually does not induce labor.
If labor is delayed, the woman may be hospitalized so that she can be monitored closely. Her temperature and pulse rate are usually recorded at least 3 times 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. Then, she may be required to stay in bed except for short showers and bathroom trips, or she may be allowed to lie or sit in bed or on the couch, but sexual activity is prohibited. In such cases, the woman should be seen by her doctor at least once a week.
Antibiotics are begun when rupture has been confirmed. Usually, antibiotics (such as erythromycin plus ampicillin or amoxicillin) are given intravenously, then by mouth for several days. They prolong the pregnancy and reduce the risk of infection in the newborn. If the membranes rupture before the 32nd week of pregnancy, corticosteroids are given to help the fetus's lungs mature.
Labor that occurs before 37 weeks of pregnancy is considered preterm.
What causes preterm labor is not well understood. However, certain conditions may make it more likely:
A healthy lifestyle during pregnancy can help, as can regular visits to the doctor or midwife, who can then identify potential problems early.
Because babies born prematurely can have serious health problems (see Problems in Newborns: Prematurity), doctors try to prevent or stop labor that begins before the 34th week of pregnancy. 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 fluid that surrounds the fetus in the uterus), 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.
Samples may be taken from the cervix, vagina, and anus to culture. Analysis of these samples may suggest the cause of preterm labor.
Drugs that can slow labor include the following:
Women may be given antibiotics until culture results are obtained. If results are negative, the antibiotics are then stopped.
If the cervix opens (dilates) more than 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) or other problems related to prematurity.
Postterm Pregnancy and Postmaturity
A postterm pregnancy is one that lasts 42 weeks or more. In postmaturity, the placenta can no longer maintain a healthy environment for the fetus because the pregnancy has lasted too long.
In most pregnancies that go a little beyond 41 to 42 weeks, no problems develop. However, beyond that time, problems may develop because the placenta often cannot continue to deliver adequate nutrients to the fetus. This condition is called postmaturity. Postterm pregnancies increase the risk of problems such as difficult labor, need for cesarean delivery, and passage of meconium (the fetus's first stool). Meconium can sometimes be inhaled before or during delivery, causing the baby to have difficulty breathing shortly after birth. In the postmature fetus, soft tissues (such as muscle) may waste away. The fetus or newborn may be deprived of oxygen, have low blood sugar, or die.
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 postterm pregnancies. Doctors use ultrasonography and may use electronic fetal heart monitoring to monitor fetal status (see Normal Labor and Delivery: Monitoring the Fetus).
Typically, at 41 weeks or sometimes at 42 weeks, labor is induced. Sometimes cesarean delivery is required.
Labor That Progresses Too Slowly
Labor that progresses too slowly may involve slow movement of the fetus through the birth canal because the fetus is too large or is abnormally positioned, the birth canal is too small, or the uterus contracts too weakly.
If labor is progressing too slowly, the fetus may be too big to move through the birth canal (pelvis and vagina). Or the fetus may be in an abnormal position. Sometimes the uterus cannot contract forcefully or often enough to keep the fetus moving.
Doctors estimate the size of the fetus and birth canal and check the fetus's position. They also check the strength and timing of contractions. These factors determine treatment.
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 delivery is necessary. If the baby is already in position to be delivered, a vacuum extractor or forceps may be used instead. If the fetus is too big, cesarean delivery is done.
Last full review/revision December 2008 by Julie S. Moldenhauer, MD