Many women who go past 40 weeks are not really post-term. Their due date was just not calculated correctly. A due date is not exact, but an estimate.

Your due date is estimated based on the first day of your last period, the size of your uterus (womb) early in your pregnancy, and with an ultrasound early in pregnancy. However:

• Many women cannot remember the exact day of their last period, which makes it hard to predict a due date.
• Not all menstrual cycles are the same length.
• Some women do not get an ultrasound early in pregnancy to establish their most accurate due date.

When a pregnancy truly is post-term and goes past 42 weeks, no one knows for sure what causes it to happen.

If you have not given birth by 42 weeks, there are greater health risks for you and your baby.

The placenta is the link between you and your baby. As you pass your due date, the placenta may not work as well as before. This could lessen the amount of oxygen and nutrients that the baby gets from you. As a result, the baby:

• May not grow as well as before.
• May show signs of fetal stress. This means the baby's heart rate does not react normally.
• May have a harder time during labor.
• Has a higher chance of stillbirth (being born dead). Stillbirth is not common but begins to increase the most after 42 weeks gestation.

Other problems that may occur:

• If the baby grows too big, it can make it harder for you to deliver vaginally. You may need to have a cesarean birth (C-section).
• The amount of amniotic fluid (water surrounding the baby) may decrease. When this happens, the umbilical cord may get pinched or pressed. This can also limit the oxygen and nutrients the baby gets from you.

Any of these problems can increase the need for a C-section.

Until you reach 41 weeks, your provider may not do anything unless there are problems.

If you reach 41 weeks (1 week overdue), your provider will do tests to check on the baby. These tests include a non-stress test and biophysical profile (ultrasound).

• The tests may show that the baby is active and healthy, and the amount of amniotic fluid is normal. If so, your provider may decide to wait until you go into labor on your own.
• These tests can also show that the baby is having problems. You and your provider must decide if labor needs to be induced.

When you reach between 41 and 42 weeks, the health risks to you and your baby become even greater. Your provider will likely want to induce labor. In older women, especially older than 40, it may be recommended to induce labor as early as 39 weeks.

When you have not gone into labor on your own, your provider will help you start. This may be done by:

• Using a medicine called oxytocin. This medicine can cause contractions to start and is given through an IV line.
• Placing medicine suppositories inside the vagina. This will help ripen (soften) the cervix and may help labor to start.
• Breaking your water (rupturing the membranes which hold amniotic fluid) can be done for some women to help labor start.
• Putting a catheter or tube in the cervix to help it begin to dilate slowly.
• Some combination of these methods may also be used by your provider.

You will only need a C-section if:

• Your labor cannot be started by your provider with the techniques described above.
• Your baby's heart rate tests show possible fetal distress.
• Your labor stops progressing normally once it has started.

Pregnancy complications - post-term; Pregnancy complications - overdue

Levine LD, Srinivas SK. Induction of labor. In: Landon MB, Galan HL, Jauniaux ERM, et al, eds. Gabbe's Obstetrics: Normal and Problem Pregnancies. 8th ed. Philadelphia, PA: Elsevier; 2021:chap 12.

Thorp JM, Grantz KL. Clinical aspects of normal and abnormal labor. In: Resnick R, Copel JA, Dugoff L, et al, eds. Creasy and Resnik's Maternal-Fetal Medicine: Principles and Practice. 9th ed. Philadelphia, PA: Elsevier; 2023:chap 40.

Updated by: John D. Jacobson, MD, Professor Emeritus, Department of Obstetrics and Gynecology, Loma Linda University School of Medicine, Loma Linda, CA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.