A full term pregnancy is 39 weeks through 40 weeks and 6 days of gestation. This definition, established jointly by the American College of Obstetricians and Gynecologists (ACOG) and the Society for Maternal-Fetal Medicine, replaced the older view that any birth between 37 and 42 weeks was simply “term.” The updated categories reflect real differences in how ready a baby is to thrive outside the womb.
The Four Categories of Term Pregnancy
What was once a single five-week window is now divided into four distinct stages:
- 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 health outcomes for babies vary meaningfully across these windows. A baby born at 37 weeks faces a different set of risks than one born at 39 weeks, even though both were previously lumped together as “term.” The distinction matters most for decisions about scheduling deliveries, cesarean sections, and inductions.
Why 39 Weeks Is the Threshold
The last few weeks of pregnancy involve a surprising amount of development. At 38 weeks, a baby’s brain weighs only about 90% of what it will at full term. Total grey matter increases by roughly 1.4% per week between 29 and 41 weeks, and white matter volume increases fivefold between 35 and 41 weeks. That means a significant portion of brain growth happens in the final stretch.
Lung maturation follows a similar pattern. The key protein that keeps the lungs’ tiny air sacs from collapsing (called surfactant) ramps up dramatically in the last weeks. Levels of this protein in the fluid surrounding the baby rise from less than 3 micrograms per milliliter at 30 weeks to more than 24 micrograms per milliliter at term. The gene responsible for producing it doesn’t even switch on until about 80% of the pregnancy is complete, and production doesn’t peak until just before birth. Those extra days between 37 and 39 weeks give the lungs critical time to finish preparing for breathing air.
Risks of Early Term Birth
Babies born in the early term window (37 to 38 weeks) are at higher risk for several complications compared to those born at 39 weeks. The most notable involve breathing. Infants delivered at 37 weeks are nearly three times as likely to experience respiratory failure and nearly three times as likely to need a ventilator compared to those delivered at 39 weeks. They also face higher rates of pneumonia and respiratory distress syndrome.
At 38 weeks, these risks drop but don’t disappear entirely. Babies born at 38 weeks show slightly elevated rates of respiratory failure and ventilator use compared to 39-week babies, though the difference is smaller. Beyond breathing, early term infants face higher rates of NICU admission, low blood sugar, and low Apgar scores (the quick health check done minutes after birth).
The effects can extend beyond infancy. Research tracking early term infants into childhood has found they perform slightly worse than full term children on academic measures. A large UK study of over 6,000 children found that early term infants scored statistically lower than full term children in four out of five school subjects by age seven. These differences are subtle, not dramatic, but they reinforce why those final days of pregnancy carry real developmental weight.
Risks of Going Past 41 Weeks
Staying pregnant longer isn’t always better, either. After 41 weeks, the placenta gradually becomes less efficient at delivering oxygen and nutrients. When the placenta can’t keep up with the baby’s needs, the baby may not grow well, may show signs of heart rate irregularities during labor, and may have a harder time tolerating the stress of delivery. This is one reason providers typically begin discussing induction as a pregnancy approaches or passes 41 weeks.
Post-term pregnancies (42 weeks and beyond) carry more significant risks, which is why very few providers will let a pregnancy continue that long without intervention.
What This Means for Scheduling Delivery
Because of the developmental differences between 37 and 39 weeks, current guidelines are clear: when both the pregnant person and the baby are healthy, elective delivery should not happen before 39 weeks. This applies to both scheduled cesarean sections and labor inductions done without a medical reason.
There’s an important nuance, though. Research has shown that inducing labor at exactly 39 weeks in healthy first-time pregnancies may actually reduce the chance of needing a cesarean section. Women induced at 39 weeks also appear to have lower rates of preeclampsia and high blood pressure compared to those who wait for labor to start on its own. This doesn’t mean every pregnancy should be induced at 39 weeks, but it does mean the conversation about timing is more nuanced than “later is always better.”
When a medical complication puts the pregnant person or baby at risk, delivery before 39 weeks is sometimes the safer choice. Conditions like severe preeclampsia, placental problems, or poorly controlled diabetes can all warrant earlier delivery. In those cases, the risks of staying pregnant outweigh the benefits of additional time in the womb.
How Gestational Age Is Counted
Pregnancy weeks are counted from the first day of the last menstrual period, not from conception. This means “39 weeks pregnant” actually reflects about 37 weeks since the egg was fertilized. An early ultrasound, typically done in the first trimester, is the most accurate way to confirm gestational age. If there’s a discrepancy between the ultrasound date and the menstrual date, providers generally go with the ultrasound estimate, since accurate dating is essential for determining whether a pregnancy has reached full term.