A full-term pregnancy lasts 39 weeks through 40 weeks and 6 days of gestation. This definition, established in 2013 by the American College of Obstetricians and Gynecologists (ACOG) and the Society for Maternal-Fetal Medicine, replaced an older system that labeled anything between 37 and 42 weeks as “full term.” The change was driven by research showing that babies born even a week or two before 39 weeks face measurably higher health risks than those who reach the full-term window.
The Four Categories of Term Pregnancy
Before 2013, the five-week stretch from 37 to 42 weeks was treated as a single category. That masked real differences in how babies fared depending on exactly when they arrived. The updated system breaks this window into four distinct labels:
- Early term: 37 weeks through 38 weeks and 6 days
- Full term: 39 weeks through 40 weeks and 6 days
- Late term: 41 weeks through 41 weeks and 6 days
- Post-term: 42 weeks and beyond
These categories exist because each carries a different risk profile for both the baby and the mother. A baby born at 37 weeks is not in the same position as one born at 39 weeks, even though both would have been called “full term” under the old system. The designations were reaffirmed as recently as 2025, and they remain the standard used by clinicians, researchers, and public health agencies.
Why 39 Weeks Is the Threshold
The last few weeks of pregnancy are more productive than they might seem from the outside. Between 37 and 39 weeks, the baby’s brain, lungs, and liver are still undergoing significant development. Research from the National Institute of Child Health and Human Development found that babies born at 37 or 38 weeks had notably poorer health outcomes compared to those born after 39 weeks, which became the foundation for redrawing the line.
The numbers are striking. Babies delivered before 39 weeks, even with confirmed lung maturity, experience respiratory problems at a rate of 7.3%, compared to just 1.6% for those delivered after 39 weeks. NICU admissions follow the same pattern: 9.6% for babies born before 39 weeks versus 3.2% for those born at or after 39 weeks. That threefold difference in intensive care admissions is why hospitals have largely stopped scheduling elective deliveries before 39 weeks unless there’s a medical reason.
What Happens After 40 Weeks
Most pregnancies don’t end on the due date. In fact, calling it a “due date” is somewhat misleading. Among first-time mothers in one large study, half gave birth by 40 weeks and 5 days, not at the 40-week mark. For mothers who had given birth before, the median was 40 weeks and 3 days. So going past your due date is not unusual at all.
That said, the risk picture does shift as pregnancies extend further. The rate of stillbirth begins to climb from 39 weeks onward, with a sharper increase after 40 weeks. By 41 weeks, the risk of unexplained stillbirth is roughly four times what it was at 39 weeks. Rates of meconium aspiration (when a baby inhales stool-stained fluid) and birth injuries also rise steadily after 38 weeks, reaching their lowest point around that time before climbing.
Post-term pregnancy, defined as 42 weeks or beyond, carries a further increase in both fetal and maternal complications. Some babies born after 42 weeks show signs of what’s called postmaturity syndrome, where the placenta has begun to function less efficiently. This can lead to restricted growth and stress on the baby. For the mother, longer pregnancies are associated with higher rates of complications during delivery.
Induction at 39 Weeks
A major clinical trial published in the New England Journal of Medicine, known as the ARRIVE trial, changed how many doctors think about full-term delivery timing. The study enrolled over 6,000 low-risk first-time mothers and randomly assigned them to either labor induction at 39 weeks or waiting for labor to start on its own.
The results surprised many in the field. Women who were induced at 39 weeks had a cesarean delivery rate of 18.6%, compared to 22.2% in the group that waited. That translates to roughly one fewer cesarean for every 28 women induced. The induced group also had lower rates of high blood pressure disorders during pregnancy. Importantly, induction at 39 weeks did not increase complications for babies. If anything, the trend favored the induction group, with adverse outcomes occurring in 4.3% of babies versus 5.4% in the expectant management group.
This doesn’t mean every pregnancy should be induced at 39 weeks, but it does mean that elective induction at full term is a reasonable option for some women, particularly first-time mothers with low-risk pregnancies. It’s a conversation worth having with your provider as you approach that window.
How Your Due Date Is Calculated
Gestational age is counted from the first day of your last menstrual period, not from conception. This means you’re already considered about two weeks “pregnant” at the time of ovulation and fertilization. A standard pregnancy is estimated at 40 weeks from that starting point, but this is an average, not a deadline.
First-trimester ultrasound is the most reliable way to confirm or adjust your due date. When the ultrasound estimate matches the date calculated from your last period, that due date is considered especially accurate. Still, even with the best dating, there’s natural variation of about two weeks in either direction. The 39-to-40-week-and-6-day window for full term accounts for this reality, defining a range rather than a single target date.