A baby is considered full term at 39 weeks and 0 days through 40 weeks and 6 days of pregnancy. This definition, established by the American College of Obstetricians and Gynecologists (ACOG) and the Society for Maternal-Fetal Medicine, replaced the older practice of calling any baby born after 37 weeks “term.” The change reflects what we now know about fetal development in those final weeks, particularly brain and lung maturation that continues right up to 39 weeks.
The Four Categories of Term Pregnancy
Rather than a single cutoff, pregnancy is now divided into more specific categories based on the week of delivery:
- 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 distinctions matter because outcomes differ meaningfully at each stage. A baby born at 37 weeks faces higher risks than one born at 39 weeks, even though both would have been called “term” under the old definition. The labels give doctors and parents a clearer picture of what to expect.
Why 39 Weeks Is the Threshold
The final four to five weeks of pregnancy are a period of rapid development, especially for the brain. Brain weight increases by roughly one-third during this stretch. The surface of the brain develops its characteristic folds, nerve fibers grow quickly, and critical connections form between the thalamus (which relays sensory information) and the outer brain. In practical terms, this means a baby born even a couple of weeks early may have less mature neurological wiring than one who reaches 39 weeks.
Lungs also continue maturing during this window. The lungs begin producing surfactant, the substance that keeps air sacs from collapsing, around 24 weeks. But adequate surfactant production for breathing without assistance doesn’t happen until much later in pregnancy. By 34 weeks, the risk of respiratory distress drops below 5%, and it continues falling through 39 weeks. Babies born at 37 or 38 weeks still have measurably higher rates of breathing problems than those born at 39 weeks.
Risks of Delivering Before 39 Weeks
Babies born in the early term window (37 to 38 weeks) are more likely to experience respiratory distress, feeding difficulties, trouble regulating body temperature, and admission to the NICU. The numbers tell a clear story: among babies born after spontaneous labor, NICU admission rates were 8.1% at 37 weeks compared to 5.1% at 39 weeks. For medically indicated deliveries, the gap was even wider, with 12.8% at 37 weeks versus 6.6% at 39 weeks.
Respiratory problems follow the same pattern. Serious breathing complications occurred in 2.7% of babies born at 37 weeks after spontaneous labor, compared to 1.3% at 39 weeks. These differences are why medical guidelines strongly discourage elective delivery before 39 weeks unless there is a medical reason to do so. When a pregnancy complication makes earlier delivery safer for the mother or baby, doctors weigh those risks against the risks of waiting.
What Happens After 41 Weeks
Staying pregnant too long carries its own risks. After 41 weeks, the placenta gradually becomes less efficient at delivering oxygen and nutrients. This can lead to reduced amniotic fluid, which increases the chance of umbilical cord compression. The baby is also more likely to pass meconium (its first stool) while still in the womb, which can cause breathing problems if inhaled during delivery.
Postterm babies (42 weeks and beyond) have higher rates of macrosomia, meaning they grow unusually large, which increases the risk of difficult delivery and shoulder injuries. About 2.5 to 10% of postterm babies are macrosomic, compared to less than 1% at term. Around 20% of postterm babies show signs of dysmaturity syndrome, where they actually appear undernourished because the placenta can no longer keep up with their needs.
Stillbirth risk also climbs with each passing week. At 37 weeks, the risk is about 2.1 per 10,000 ongoing pregnancies. By 41 weeks it reaches 6.1, and at 42 weeks it jumps to 10.8 per 10,000. This is why most providers will recommend induction or increased monitoring once a pregnancy passes 41 weeks.
Induction at 39 Weeks
A large clinical trial known as the ARRIVE trial looked at what happens when low-risk first-time mothers are induced at exactly 39 weeks compared to waiting for labor to start on its own. The results showed no significant difference in newborn complications between the two groups (4.3% in the induced group versus 5.4% in the expectant group). Cesarean delivery rates were actually 16% lower in the induced group: 18.6% compared to 22.2%.
This doesn’t mean every pregnant person should be induced at 39 weeks. It does mean that if your provider offers induction at 39 weeks, it is a reasonable option that doesn’t increase risk to your baby and may lower your chance of needing a cesarean. The decision is personal and depends on your specific situation and preferences.
How Your Due Date Is Calculated
All of these week-based categories depend on knowing how far along you are, which makes accurate pregnancy dating important. Gestational age is counted from the first day of your last menstrual period, not from conception. A first-trimester ultrasound is the most reliable way to confirm or adjust your due date, and on average it differs from last-menstrual-period dating by less than one day.
If your early ultrasound and your period-based date disagree by more than a few days, your provider will typically go with the ultrasound measurement. Getting this right matters because a difference of even one week can shift which category your baby falls into at delivery, and that can affect decisions about whether to induce, wait, or schedule a cesarean.