What is a post-term pregnancy?

A pregnancy that lasts more than 42 weeks (294 days since the first day of the last menstrual period) is considered post-term. The vast majority of women deliver between 37 and 42 weeks of pregnancy. Other terms often used for this include prolonged pregnancy, post-dates pregnancy (after the due date), and postmaturity.

What causes post-term pregnancy?

It is not known why some women carry a pregnancy longer than others. It is often due to a miscalculation of pregnancy conception dates. A woman is much more likely to have a post-term pregnancy if previous pregnancies went beyond 42 weeks.

Why is post-term pregnancy a concern?

Post-term pregnancy is associated with longer labors and operative delivery (forceps or vacuum-assisted birth). Mothers are at increased risk for vaginal birth trauma due to a large baby. Cesarean delivery is twice as likely in a post-term pregnancy because of the size of the baby. Mothers are also at increased risk for infection and wound complications, and postpartum (after birth) hemorrhage.

There are also risks for the fetus and newborn in a post-term pregnancy, including stillbirth and newborn death. Toward the end of pregnancy, the placenta, which supplies the fetus with the nutrients and oxygen from the mother's circulation, begins to age and may not function as efficiently as before. Amniotic fluid volume may decrease and the fetus may stop gaining weight, or may even lose weight. Risks can increase during labor and birth for a fetus with poor oxygen supply. Birth injury may also occur if the baby is large. Babies born after 42 weeks may be at risk for meconium aspiration, when a baby breathes in fluid containing the first stool. Hypoglycemia (low blood sugar) can also occur because the baby has too little glucose-producing stores.

How is post-term pregnancy diagnosed?

Correct pregnancy dating is important in accurately diagnosing and managing post-term pregnancy. The size of the uterus at various points in early pregnancy, the date the fetal heartbeat was first heard, and when a mother first feels fetal movement all help confirm pregnancy dates. Ultrasound (a diagnostic imaging technique that uses high-frequency sound waves and a computer to create images of blood vessels, tissues, and organs) is often used in early pregnancy to establish or confirm a due date. 

In a post-term pregnancy, testing may be done to check fetal well-being and identify problems. Tests often include ultrasound, nonstress testing (how the fetal heart rate responds to fetal activity), and estimation of the amniotic fluid volume.

Management of post-term pregnancy

The goal of management of post-term pregnancy is to prevent complications and deliver a healthy baby. Specific management for post-term pregnancy will be determined by your doctor based on:

• Your pregnancy, overall health, and medical history

• Extent of the condition

• Your tolerance for specific medications, procedures, or therapies

• Expectations for the course of the condition

• Your opinion or preferences

Maternal and fetal testing are often performed for a post-term pregnancy to monitor for signs of problems. Some of the ways to detect potential problems include the following:

• Fetal movement counting. Keeping track of fetal kicks and movements. A change in the number or frequency may mean the fetus is under stress.

• Nonstress testing. A test that watches the fetal heart rate for increases with fetal movements, a sign of fetal well-being.

• Biophysical profile. A test that combines the nonstress test with an ultrasound to evaluate fetal well-being.

• Ultrasound. A diagnostic imaging technique that uses high-frequency sound waves and a computer to create images of blood vessels, tissues, and organs. Ultrasounds are used to view internal organs as they function, and to assess blood flow through various vessels. Ultrasounds are used to follow fetal growth.

• Doppler flow studies. A type of ultrasound that uses sound waves to measure blood flow, commonly used in intrauterine growth restriction (IUGR) babies.

If tests determine that it is no longer healthy for the fetus to stay in the mother's uterus, labor may be induced, to deliver the baby.

The decision to induce labor for post-term pregnancy depends on many factors. During labor, the fetal heart rate may be monitored with an electronic monitor to help identify changes in the heart rate due to low oxygenation. Changes in a baby's condition may require a cesarean delivery.

During labor, continuous fetal heart rate monitoring is often used to help detect changes in the fetal heart rate. Because a post-term fetus is more likely to pass meconium (the first stool) during labor, the risk of meconium aspiration is increased. The baby may need suctioning and special care after delivery.

Amnioinfusion is sometimes used during labor if there is very little amniotic fluid or the fetus is compressing the umbilical cord. In amnioinfusion, a sterile fluid is instilled with a catheter (hollow tube) into the broken amniotic sac to help replace the low levels of fluid and cushion the fetus and cord.

If labor does not progress or there is fetal distress, cesarean delivery may be needed. Very large babies may have difficulty at delivery, and forceps or vacuum-assisted delivery may be needed. Clinicians must proceed with caution to prevent any harm to the baby.