Labor, Delivery, and Postpartum Period

Full-term babies are delivered sometime between 37 and 42 weeks of pregnancy. (Those weeks are counted from the first day of your last menstrual period, or LMP.) 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.

Some post-term pregnancies are not truly post-term. A common "cause" is an incorrect due date. (Your due date is 40 completed weeks after your LMP. 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.

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

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 risk increases with each week, up to 10 per 1,000 post-term pregnancies after 43 weeks.⁹ This is why your doctor or nurse-midwife will monitor your baby after 40 to 41 weeks.

Many health professionals 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:⁹

• Any time after the due date that a fetal problem shows up in testing, it is time to deliver.
• For a cervix that is "favorable" for delivery—is softening, thinning, or opening—many doctors speed up the process by inducing labor. This may start in the doctor's office with a simple sweeping of the membranes. Watching and waiting until 42 weeks is also a reasonable choice, as long as there are no signs of problems. (There is no research that shows one choice to be better than the other for mothers and babies.⁹ Discuss this with your health professional.)
• For a cervix that has not started to soften, thin, or open, watching and waiting is a reasonable choice. But giving medication to soften the cervix and induce labor does seem to have some advantages. A review of studies has shown that softening and inducing labor after 41 completed weeks lowers the rate of stillbirths and infant deaths (though, either way, deaths are very rare).¹⁰ And the rate of cesarean delivery, pain medicine use, and forceps or vacuum delivery does not increase.⁹

For safety reasons, most health professionals will plan to deliver a baby by 42 weeks, inducing labor if necessary. Generally, the risks of waiting for natural labor beyond 42 weeks are thought to outweigh the benefits.