The date your baby is due—your estimated due date (EDD)—is calculated from the first day of your last menstrual period (LMP). The EDD is used as a guide for checking your pregnancy’s progress and tracking the growth of the fetus.
An ultrasound exam often is used to confirm the due date. Your obstetrician–gynecologist (ob-gyn) will evaluate the dating from your ultrasound exam and compare it with your due date based on your LMP. Once a due date has been selected, it does not change no matter how many additional ultrasound exams you may have during your pregnancy.
The average length of pregnancy is 280 days, or 40 weeks, counted from the first day of your LMP. A pregnancy that lasts 41 weeks up to 42 weeks is called “late term.” A pregnancy that lasts longer than 42 weeks is called “postterm.”
The causes of postterm pregnancy are unknown, but there are several factors that may increase your chances of having a postterm pregnancy. These factors include the following:
The health risks for you and your fetus may increase if a pregnancy is late term or postterm, but problems occur in only a small number of postterm pregnancies. Most women who give birth after their due dates have uncomplicated labor and give birth to healthy babies. Risks associated with postterm pregnancy include the following:
Other risks include an increased chance of an assisted vaginal delivery or cesarean delivery. There also is a higher chance of infection and postpartum hemorrhage when your pregnancy goes past your due date.
A pregnancy between 40 weeks and 41 weeks of gestation does not necessarily require testing, but at 41 weeks your ob-gyn or other health care professional may recommend testing. These tests may be done weekly or twice weekly. The same test may need to be repeated or a different test may need to be done. In some cases, delivery may be recommended.
Tests of fetal well-being use electronic fetal monitoring and sometimes ultrasound. During electronic fetal monitoring, two belts are placed around your abdomen to hold sensors. These sensors measure fetal heart rate and the frequency of uterine contractions. These tests are done in your ob-gyn or other health care professional’s office or hospital. Tests may include nonstress tests, biophysical profiles, and contraction stress tests.
The nonstress test (NST) measures the fetus’s heart rate for a specific period of time, usually 20 minutes. Results of the NST are noted as reactive (reassuring) or nonreactive (nonreassuring). A nonreactive result does not necessarily mean that the fetus is not healthy. Nonreactive nonstress test results often are followed by other tests to give more information.
A biophysical profile (BPP) involves monitoring the fetal heart rate as well as an ultrasound exam. It checks the fetal heart rate, breathing, movement, and muscle tone. The amount of amniotic fluid also is assessed.
A contraction stress test (CST) assesses how the fetus’s heart rate changes when the uterus contracts. To make your uterus contract mildly, you may be given oxytocin through an intravenous (IV) tube in your arm. Results are noted as reassuring or nonreassuring. Results also can be equivocal (the results are not clear) or unsatisfactory (there were not enough contractions to produce a meaningful result).
Labor induction may be recommended if your pregnancy reaches 41 weeks. Induction is started using medications or other methods. To induce labor, your cervix needs to have started softening in preparation for delivery. This is called cervical ripening. Medications or other methods may be used to start this process.
Methods for inducing labor may include the following:
The risks of labor induction may include changes in fetal heart rate, infection, and contractions of the uterus that are too strong. You and your fetus will be monitored throughout the process. Another possibility is that labor induction may not work. The method used to induce labor may need to be repeated. In some cases, you may need to have an assisted vaginal delivery or a cesarean delivery.
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