Topic Overview

Most babies are born at 37 to 42 weeks of pregnancy. (Those weeks are counted from the first day of your last menstrual period.) A pregnancy that has reached 42 or more weeks is called a "post-term" or "post-date" pregnancy. You might also call it "overdue." Pregnancy that lasts beyond the due date is fairly common.

In most cases, there is no obvious cause of a post-term pregnancy.

Some post-term pregnancies are not truly post-term. The due date may not have been figured correctly. (Your due date is 40 completed weeks after your last period. If you ovulated late in your cycle, your pregnancy didn't start as early as this due date says.)

An ultrasound measurement of your fetus during the first trimester can give the most accurate due date. But even that due date is an estimate of when you might deliver.

What concerns are linked to post-term pregnancy?

Most often, a post-term baby is born in good health. But a very small number of post-term pregnancies are linked to stillbirth and infant death. This is why your doctor or midwife will watch you closely after 40 to 41 weeks.

Many doctors and midwives want to lower risks for the post-term baby by delivering by or before 42 weeks. In most cases, watching and waiting is also fine. It is often hard to know which choice is best during the 2 weeks after the due date:

• Any time after the due date that a fetal problem shows up in testing, it is time to deliver.
• If the cervix is ready for delivery, many doctors speed up the process by inducing labour. This may start in the doctor's office with a simple sweeping of the membranes.
• If the cervix isn't ready for delivery, watching and waiting is a reasonable choice. But giving medicine to soften the cervix and induce labour does seem to have some advantages. A review of studies has shown that softening the cervix and inducing labour after 41 completed weeks lowers the rate of stillbirths and infant deaths (though, either way, deaths are very rare).^{footnote 1}

For safety reasons, most health professionals will plan to deliver a baby by 42 weeks, inducing labour if necessary.