A full term pregnancy lasts between 39 weeks, 0 days and 40 weeks, 6 days of gestation. That window spans roughly one week before your due date to one week after it. 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 lumped all births from 37 to 42 weeks under a single “term” label. The change happened because research made clear that every week of pregnancy matters for a baby’s health.
Why the Definition Changed
Before 2013, any pregnancy lasting 37 to 42 weeks was simply called “term.” That five-week range was treated as though all outcomes were roughly the same. But mounting evidence showed they weren’t. Babies born at 37 weeks faced meaningfully different risks than babies born at 39 or 40 weeks, particularly when it came to breathing problems, blood sugar regulation, and brain development. To reflect those differences, a work group convened in late 2012 and split the old “term” category into four distinct labels:
- Early term: 37 weeks, 0 days through 38 weeks, 6 days
- Full term: 39 weeks, 0 days through 40 weeks, 6 days
- Late term: 41 weeks, 0 days through 41 weeks, 6 days
- Postterm: 42 weeks, 0 days and beyond
These aren’t just labels. They guide real clinical decisions about when it’s safe to schedule a delivery and when it’s better to wait.
What Happens in the Final Weeks of Pregnancy
The reason those last few weeks carry so much weight comes down to what’s still developing inside the baby. From about week 35 onward, the fetus enters its most rapid period of weight gain, adding roughly 8 to 12 ounces per week. Fat deposits build up across the body, especially around the shoulders, helping the baby regulate temperature after birth.
But the most consequential changes are happening in the brain. During the final four to five weeks of pregnancy, brain weight increases by approximately one third. The brain’s surface folds dramatically multiply, and the connections between deeper brain structures and the outer cortex begin forming in earnest. Nerve fibers responsible for transmitting signals grow rapidly during this window. This is not a minor finishing touch. It represents a large portion of the brain’s overall wiring and development.
The lungs are also completing a critical transition. Fetal lungs move through five developmental stages, and the final “alveolus stage,” when the tiny air sacs that handle oxygen exchange mature, doesn’t begin until around week 36. Full maturation continues through 39 weeks and beyond, which is why breathing problems are one of the most common complications in babies born even a couple of weeks early.
Why Early Term Is Not the Same as Full Term
Babies born at 37 or 38 weeks often do well, but statistically they face higher rates of complications than babies born at 39 or 40 weeks. One hospital-based study found that 11.5% of early term infants required intensive care admission, compared with 7.9% of full term infants. Respiratory distress, a condition where the baby struggles to breathe on its own, and blood sugar instability are more common among early term babies, and both decrease in frequency with each additional week of gestation.
The differences extend beyond the newborn period. Multiple studies have found that early term children score lower across a range of cognitive and educational measures compared to their full term peers. The gaps are not dramatic for any individual child, but at a population level they are consistent enough that medical organizations now recommend against scheduling deliveries before 39 weeks unless there is a specific medical reason.
The 39-Week Guideline for Scheduled Deliveries
Current guidelines are clear: when a pregnant person and their baby are healthy, induction or cesarean delivery should not be scheduled before 39 weeks. Babies born at or after 39 weeks have the best chance at healthy outcomes.
For healthy first-time pregnancies with a single baby, your provider may discuss the option of induction at 39 weeks. Research suggests that inducing labor at 39 weeks in this specific group may actually reduce the risk of cesarean birth. It may also lower rates of preeclampsia and gestational hypertension compared to waiting for labor to start on its own. This doesn’t mean induction at 39 weeks is recommended for everyone, but it’s a conversation worth having with your provider if it applies to your situation.
When a medical condition puts you or your baby at risk, your provider may recommend delivery before 39 weeks. The guidelines are specifically about avoiding non-medically indicated early deliveries.
What Happens After 41 Weeks
Going past your due date is common and usually not dangerous. Most people who deliver after 40 weeks have uncomplicated labor and healthy babies. But the risks do increase gradually as a pregnancy moves into late term (41 weeks) and especially postterm (42 weeks and beyond).
The main concerns with postterm pregnancy involve the placenta, which can become less efficient at delivering oxygen and nutrients. Amniotic fluid levels may drop, which can compress the umbilical cord. The baby may also continue growing larger, a condition called macrosomia, which increases the chance of a difficult delivery, assisted vaginal birth, or cesarean. In rare cases, the baby may pass stool (meconium) into the amniotic fluid, which can cause serious breathing problems if inhaled. The risk of stillbirth, while still small in absolute terms, also rises after 42 weeks.
For the mother, postterm pregnancy carries a higher chance of infection and heavy bleeding after delivery. For these reasons, most providers will discuss induction if you haven’t gone into labor by 41 weeks, and will typically recommend it before 42 weeks.
Changes You May Notice Near Full Term
As you approach 39 weeks, your body is preparing for labor in ways you can feel. Your cervix gradually thins and softens, a process called effacing, which helps the birth canal open when labor begins. You may notice your breasts leaking colostrum, a thick, yellowish pre-milk that will be your baby’s first food. Many people experience increased pelvic pressure as the baby drops lower, along with more frequent and sometimes stronger practice contractions. These physical shifts are signs that your body is doing exactly what it’s supposed to do in the final stretch of a full term pregnancy.