Post-Term Pregnancy

What Causes It and Who's at Risk?

In about 80 percent of all pregnancies, birth takes place between 38 and 42 weeks. About half of the remainder, or 10 percent, are preterm (end before 37 weeks), and the other 10 percent or so last beyond 42 weeks. These latter pregnancies — those lasting beyond the end of the 42nd week — are considered to be post-term.

Many of these pregnancies, however, may turn out not to be post-term after all. Often, a miscalculated due date is responsible for a pregnancy being considered post-term. When early ultrasound testing is used to confirm the due date, the actual frequency of post-term pregnancy turns out to be about 2 percent of all pregnancies. The causes of post-term pregnancy are largely unknown. Heredity and hormonal factors may have an effect.

Concerns in a post-term pregnancy center on the risks posed to the baby. After 41 weeks, the amount of amniotic fluid inside the uterus may decrease dramatically. This can increase the risk that the umbilical cord will become compressed during labor or delivery, interrupting the flow of oxygen to the baby. Post-term pregnancies also increase the risk of meconium in the amniotic fluid. Meconium is the fetus's stool, and its presence means that the baby has had its first bowel movement while in the uterus. A type of pneumonia may develop if the baby inhales the meconium into the lungs while still in the uterus. For this reason, your doctor will suction the nose, mouth, and back of the baby's throat as soon as the head is delivered. A pediatrician or other caregiver will then immediately pass a tube into the baby's windpipe to quickly suction out the meconium before it has a chance to reach the baby's lungs. You might not hear your baby cry until after this suctioning has been completed.

Another concern in a post-term pregnancy is macrosomia, or a baby weighing more than 4,500 grams (9 pounds 14 ounces). Such large babies may have a hard time getting safely through the birth canal during delivery. This is one of the reasons why cesarean birth is more common in post-term pregnancies. But despite the increased risks to the baby in a post-term pregnancy, most of these babies are born safely with careful management.

How Is It Managed?

If your pregnancy progresses beyond 41 or 42 weeks, one of your doctor's first concerns will be to find out whether the due date is accurate. Going back over the findings of previous exams and tests will help her or him pin down the true length of gestation. Knowing when you first felt the baby move, when the first fetal heart sounds were heard, how well the size of the baby correlated with the date of the pregnancy, the height of the uterus at 20 weeks (normally at the level of the mother's navel), and the results of early ultrasound exams all provide measures of how far along gestation was at various points during the pregnancy.

If your doctor determines that your pregnancy is truly post-term, the approach she or he takes will depend on your individual circumstances. Tests to find out the condition of the fetus, such as non-stress tests, contraction stress tests, or biophysical profiles, will yield useful information. An ultrasound exam will be used to determine how much amniotic fluid surrounds the fetus. At signs that the baby's condition may be worsening, or that the amniotic fluid volume is low, the decision will be made to deliver the baby.

In addition, the cervix may be checked weekly after 40 weeks to find out whether it is beginning to dilate. Many doctors decide to induce labor when the cervix becomes “ripe” (softened, effaced, and starting to dilate) after 41 weeks. In a woman whose cervix has not yet begun to dilate, but in whom delivery is the best course, agents can be used to ripen the cervix. These include gels containing the hormone prostaglandin or small inserts called laminaria, which are placed inside the cervix and expand as they absorb moisture.

Many doctors may adopt a wait-and-see attitude if the cervix is not dilated and there are no signs that the baby is in danger. Others feel it's best to deliver a post-term baby if labor has not begun by the end of 42 weeks, regardless of the condition of the cervix. Generally, a pregnancy will not be allowed to go beyond 43 or 44 weeks, because the risks to the baby are significantly increased after that time.

If delivery is decided on, how the baby is born — vaginally or by cesarean — will depend on many factors. A baby who is too large to pass through the mother's pelvis must be born by cesarean. A woman whose cervix is ripe and whose baby has shown no signs of problems is a candidate for vaginal delivery. The baby's heart rate, as well as contractions of the mother's uterus, will be monitored closely during a vaginal birth of a post-term infant. A cesarean birth may be necessary if there are signs that the baby is not tolerating the stress of labor.

Post-term babies may have long, thin bodies, without the whitish coating of vernix found on normal newborns. Because of the longer time they've spent in the uterus, they are frequently born with long fingernails, lots of hair, and wrinkled palms and soles. What about the future? Even with the risks of post-term pregnancy, most post-term babies come safely into the world. Your doctor will help you weigh the benefits and risks of the available options to handle post term pregnancy. Despite the risks to the baby in a post-term pregnancy, the long-term outlook for most post-term babies is excellent.