A pregnancy is not full term until 39 weeks, according to a new set of definitions endorsed by the American Congress of Obstetrics and Gynecologists (ACOG) and the Society for Maternal-Fetal Medicine. This represents an important change from the old understanding under which pregnancy was considered full term from 37 weeks to 42 weeks.

The new set of definitions breaks down like this:

• Early term: 37 weeks to 38 weeks and 6 days
• Full term: 39 weeks to 40 weeks and 6 days
• Late term: 41 weeks to 41 weeks and 6 days
• Postterm: 42 weeks and beyond

In practical terms, what this means is that whereas for years mothers have been told that if they make it to 37 weeks they can assume their babies are ready to be born, those same babies will now be considered not yet full term for another two weeks.

Why Change the Definitions?

The change matters because, as the March of Dimes has been arguing and actively educating the public on, elective deliveries performed before 39 weeks without a medical reason consistently result in greater risks of health problems for the baby. These risks include problems with breathing as well as developmental issues.

Nevertheless, these early births—via induction or cesarean section—have risen to as many as 15 percent of births annually in the United States. The rate of early delivery is so high because the risks are not widely understood, because births are scheduled for reasons of health-care provider convenience, and because estimated due dates are often miscalculated, resulting in the mistaken belief that a baby is to term when it really is not yet.

The new definitions should encourage physicians to practice greater patience around the end of pregnancy. In most cases, spontaneous labor is the best way to determine the healthiest time for birth. It does, however, remain the case that there are some health conditions for which early induction is medically indicated, but in general labor should be allowed to start on its own.

The new definitions are good news not only for babies, but also for mothers. Not only is carrying to full term less likely to pose the health problems associated with prematurity in the baby, but the possibility of more spontaneously initiated labors would mean fewer labors via induction or surgery. Both of these interventions are accompanied by health risks to the mother, including more painful contractions, risk of infection, uterine rupture, and the possibility of cascading interventions in which ultimately induction will not work and the major surgery that is cesarean section will become necessary.

Part of a Larger Trend

The new definitions seem to be in line with a shift signaled by, among other things, a blog post published in March by ACOG President James T. Breeden in which he argued that “a vaginal birth that occurs after the natural onset of labor is ideal” and that both cesarean sections and labor induction should be limited to cases of medical necessity.

As Miriam Pérez commented at Rewire in April, “I’m glad to see ACOG taking such a bold stance in supporting ‘mother nature’s’ role in the beginning of labor, but I know that it may take a long time for these recommendations to actually affect the use of inductions and preterm c-sections as parts of standard obstetrical practice.”

The new definitions hopefully will be a catalyst for a cultural shift toward encouraging labor to begin on its own.