Waiting for the birth of a child can be both an exciting and anxious time, especially if your pregnancy extends past the due date. The average length of pregnancy is 280 days, or 40 weeks. Most women give birth between 38 weeks and 41 weeks of pregnancy. A pregnancy that lasts longer than 42 weeks is called “postterm.” About 6 out of 100 women give birth at 42 weeks or later.
Your Due Date
The date your baby is due—your due date—is calculated from the first day of your last menstrual period (LMP). The due date is used as a guide for checking your pregnancy’s progress and tracking the growth of the fetus. It is only an estimate of when your baby will be born.
An ultrasound exam may be performed to help confirm the age of the fetus. This exam is most accurate for setting the due date when it is done before 20 weeks of pregnancy. It is less reliable when performed later in pregnancy. This is one reason why early prenatal care is important.
Causes of Postterm Pregnancy
The most common cause of postterm pregnancy is an error in calculating your due date. For some women, it may be difficult to recall when their last menstrual period occurred. If the LMP is incorrect, the due date also will be incorrect. When a postterm pregnancy truly exists, the cause usually is unknown.
Risks of Postterm Pregnancy
Health risks for the baby and mother increase if a pregnancy is prolonged past 42 weeks. The more prolonged the pregnancy, the greater the risks. However, problems occur in only a small number of postterm pregnancies. Most women who give birth after their due dates have healthy newborns.
After 42 weeks, the placenta may not work as well as it did earlier in pregnancy. Also, later in pregnancy, the amount of amniotic fluid often begins to decrease. Less fluid may cause the umbilical cord to become pinched as the baby moves or as the uterus contracts.
If pregnancy goes past 42 weeks, a baby has an increased risk of certain problems. These problems include dysmaturity syndrome, macrosomia, meconium aspiration, fetal injury, and stillbirth. Women who are pregnant after 42 weeks are at increased risk of problems during labor and cesarean birth.
Tests for Fetal Well-Being
When a baby is not born by the due date, tests may help your health care provider check on the baby’s condition. Keep in mind that no test can provide 100% assurance. These tests cannot always find a problem even if one exists, or the results may show that there may be a problem when one does not exist.
Fetal movement counting (sometimes called “kick counts”) is a test that can be done on your own at home. Healthy babies generally tend to move the same amount each day. A kick count is a record of how often you feel your baby move. Your health care provider will explain how to do a kick count. The usefulness of kick counts in reducing the risk of serious problems is not clear. Some health care providers do not use kick counts for this reason.