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Pregnancy

How Many Weeks is a “Full-term Pregnancy”?

Jennifer Lincoln, MD, IBCLC, Board Certified OB/GYN
January 3, 2019 . 2 min read

In October 2013, the American College of Obstetricians/Gynecologists (ACOG) issued an updated definition for term pregnancies. The updated definitions are significant because they change when pregnancies are classified as “full term.”

Prior to the new definition, a pregnancy was deemed full term from three weeks before to two weeks after a woman’s due date; that is, a full term pregnancy ranged from 37-42 weeks. It was thought that babies born in this window all performed equally well and had the same level of risk for complications.

Based on newer global data, however, we now know that this isn’t the case. Babies born between 39 and 41 weeks have the least amount of complications, whereas babies born earlier and later than that have some increased risks. This is what has led ACOG to adjust their terminology.

Pregnancies should now be defined as follows:

Early term: 37 weeks through 38 weeks

Full term: 39 weeks through 40 weeks

Late term: 41 weeks

Postterm: 42 weeks and late

The goal of the new definitions is to help patients and doctors understand that babies who are not yet 39 weeks still have developing and maturation to do—and that is best done in the womb.

Elective deliveries for non-medical reasons, like a doctor going away on vacation or a parent wanting a certain birthday, are not appropriate before the 39-week mark. While that is not a new concept, ACOG hopes that labeling these babies as early term will discourage elective early deliveries.

Reviewed by Dr. Jen Lincoln, December 2018

Sources:

  • American Congress of Obstetricians/Gynecologists
  • Committee Opinion #579
  • Definition of term pregnancy.
    Spong CY
  • Defining “term” pregnancy: recommendations from the Defining “Term” Pregnancy Workgroup
  • JAMA 2013;309:2445–6.

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