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Sometimes, a pregnancy may seem “too long” because the original due date was wrong. This can happen when you do not know the date of your last period or had long or irregular menstrual cycles. An early ultrasound is usually a better indicator of the baby’s gestational age than from your last period.

James W. Brann, MD

Your Pregnancy MD
Post-Term or Over Due

Post-term pregnancy

Post-term pregnancy happens when the baby is not born by 42 weeks gestation. About 10% of pregnancies last longer than 42 weeks. The weeks are counted from the first day of the mother’s last menstrual period. The average length of a pregnancy is about 40 weeks, and the vast majority of births take place between 37 and 42 weeks.

Sometimes, a pregnancy may seem “too long” because the original due date was wrong. Inaccurate dating is the most common cause of post-term pregnancy.[Ref;] This can happen when the woman did not know the date of her last period or had long or irregular menstrual cycles. When this is the case, most women get an ultrasound in the first trimester of pregnancy. Early ultrasounds can help a doctor determine how far along the pregnancy is. The new due date from an early ultrasound is usually a better indicator of the baby’s gestational age than an ultrasound later in pregnancy.

Babies are usually very large if they are post-term.
Large post-term baby

If the gestational age is correct, and the pregnancy is truly longer than 42 weeks, most doctors will run a few tests to track the health of the baby and the safety of the mother

What should you expect with an overdue pregnancy

Who is at Risk for a Post-term Pregnancy?

Doctors do not know the main cause of post-term pregnancies. You are at a higher risk of having a pregnancy longer than 42 weeks, though, if any of the following characteristics apply to you:

  • This is your first pregnancy.
  • You have had a post-term pregnancy in the past.
  • You were born post-term.

What are the Risks of a Post-term Pregnancy?

A term pregnancy is good for the baby because it allows time to grow and develop. In post-term pregnancies, this can mean the baby has time to grow too big to fit through the birth canal. In addition, a post-term pregnancy can make it hard for the baby to get enough nutrients and support from your body. If the baby is too big or is not growing safely, this can create problems for both the mother and the baby. Other risks are listed below.

Risks to the mother:

  • difficulty having a vaginal birth
  • tears to the vagina, labia, or rectum during vaginal birth
  • all risks associated with a C-section (like bleeding and infection) if surgery is needed

Risks to the baby:

  • restricted growth (from problems with the placenta or amniotic fluid)
  • injuries during vaginal birth (like broken bones or pinched nerves) because of being big
  • breathing problems at birth (from having a bowel movement and breathing it in through the amniotic fluid)

Size and growth problems can lead to other problems, like emergency cesarean deliveries (C-sections) or even stillbirth.

What Types of Tests are Done for Post-term Pregnancies?

Because there are some risks to carrying the baby too long, your doctor will probably run several tests once you are 41 or 42 weeks along. They will probably do the following tests, some of which require monitoring over time:

  • the nonstress test, to monitor the baby’s heartbeat
  • the contraction stress test, to monitor the baby’s reaction to a medicine that induces contractions
  • a Biophysical Profile (BPP) score, to describe five test results (the baby’s movements, breathing, arm and leg stretches, amniotic fluid levels—each seen in an ultrasound—and the nonstress test results)

Depending on the results of these tests, your doctor will let you know whether it is safe to let the pregnancy continue. If not (because, for instance, the baby has a slow heartbeat or isn’t growing), your doctor may recommend that labor be induced or that a C-section be arranged.

What Can You Expect Labor and Delivery with a Post-term Pregnancy?

If you do not naturally go into labor by 42 weeks, your doctor may want to induce labor. Vaginal birth will only be recommended if the baby does not seem to be too large or in immediate danger. Medicine can help get your body started by encouraging contractions and the opening of your cervix. You will be monitored closely during this process to make sure the baby can still be born safely without surgery.

If you would prefer a cesarean delivery, you should discuss this plan with your doctor. In some cases, your healthcare provider may recommend that you have a C-section instead of trying a vaginal birth. A cesarean delivery may be needed if the baby’s estimated weight is over 10 pounds, if the attempted natural labor takes too long, or if the monitors detect problems with your baby’s heartbeat or movements.

Once the baby is born, he or she may have certain features. When babies are in the womb longer than expected, they may have the following traits:

  • long, thin arms
  • long hair and nails
  • dry, flaky skin (that may or may not be discolored)
  • loose skin, especially on the legs and buttocks
  • wide, alert eyes

After a few weeks the differences between your baby and any other baby born at term will be less noticeable. There are no known long-term negative effects of being born at or after 42 weeks.

Coming to Terms with Post-term Pregnancy

If you have been pregnant for 42 weeks or longer, work closely with your healthcare provider to decide what the best choice is for delivering your baby. Whether you let the pregnancy run its course or opt for a C-section, you can rest easy knowing that most post-term pregnancies have happy, healthy endings

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