A pregnancy reaching 39 weeks is considered full term, and this is the point when birth carries the lowest risk for both mother and baby. Before 39 weeks, a baby’s brain, lungs, and liver are still finishing critical development. After 41 weeks, the risk of complications begins to climb. The sweet spot for the safest outcomes falls between 39 weeks and 40 weeks and 6 days.
How Pregnancy Weeks Are Classified
The medical community breaks the final stretch of pregnancy into specific categories, and the labels matter more than you might expect. Early term covers 37 weeks through 38 weeks and 6 days. Full term runs from 39 weeks through 40 weeks and 6 days. Late term is the narrow window of 41 weeks through 41 weeks and 6 days, and postterm begins at 42 weeks.
These distinctions replaced the older idea that any birth after 37 weeks was simply “term.” That outdated thinking led to a wave of elective early deliveries that turned out to carry real, measurable risks for newborns. The updated classifications reflect what we now know about how much development happens in those final two to three weeks.
What Happens in the Final Weeks of Development
At 35 weeks, a baby’s brain weighs only about two-thirds of what it will at birth. The entire ninth month is dedicated to finishing brain growth and adding protective body fat. Week by week, the changes are significant: bones harden, the coating that protects the skin thickens, fine body hair falls away, and toenails grow to the tips of the toes. At 38 weeks, the baby is still gaining roughly half a pound per week.
The lungs are one of the last organs to fully mature. A baby born at 37 weeks may look healthy and full-sized but can still struggle with breathing, temperature regulation, and feeding because these systems aren’t quite finished. By 39 weeks, these processes are complete and the baby is genuinely ready for life outside the womb.
Why 37 Weeks Is Riskier Than It Sounds
Being born at 37 weeks is survivable and common, but it’s not the same as being born at 39 weeks. The NICU admission rate at 37 weeks is 8.4%, and within that week, the risk drops sharply day by day. Babies born at exactly 37 weeks and 0 days have a 12.7% chance of needing intensive care, while those born at 37 weeks and 6 days face a 5.2% chance. That steep decline in just seven days shows how rapidly a baby matures during this period.
Babies born at 37 or 38 weeks are more likely to have trouble maintaining body temperature, feeding effectively, and keeping blood sugar stable. These issues are usually temporary, but they can mean a longer hospital stay, separation from parents, and a more stressful start.
Long-Term Outcomes for Early Term Babies
Parents often worry about whether being born a couple of weeks early will affect their child down the road. A large study of nearly 800,000 siblings found reassuring results: cognitive outcomes in adolescence did not meaningfully differ between children born at 34 through 39 weeks and those born at 40 weeks. The difference amounted to less than 1 IQ point. Significant cognitive deficits only appeared in children born before 34 weeks, where IQ reductions of 2 to 4 points were measurable.
So while the short-term medical risks of an early term birth are real, the long-term developmental picture is encouraging for babies born in the 37 to 38 week range.
What Happens When Pregnancy Goes Past 41 Weeks
Going past your due date is common, but staying pregnant well beyond 41 weeks carries increasing risk. The rate of stillbirth and early newborn death at 42 weeks is roughly double what it is at 40 weeks, rising from about 2 to 3 per 1,000 deliveries to 4 to 7 per 1,000. At 43 weeks that risk quadruples, and at 44 weeks it increases five to sevenfold.
Other complications also climb after 41 weeks. The baby is more likely to pass its first stool before birth, which can be inhaled into the lungs. The placenta also becomes less efficient at delivering oxygen and nutrients the longer a pregnancy continues past term. For these reasons, most providers will discuss induction of labor between 41 and 42 weeks, even in otherwise uncomplicated pregnancies.
When Delivering Before 39 Weeks Is the Right Call
The 39-week guideline applies to healthy, uncomplicated pregnancies. A number of medical conditions make earlier delivery safer than waiting. These include severe high blood pressure or preeclampsia, poorly controlled diabetes, problems with the placenta, very low amniotic fluid, restricted fetal growth, and premature rupture of membranes.
In these situations, the risks of continuing the pregnancy outweigh the risks of an earlier birth. Your provider will weigh the specific condition against the baby’s gestational age to find the best timing. Lung maturity testing sometimes plays a role in these decisions, but if there’s a clear medical need, delivery proceeds regardless of those results.
The Safest Window, in Practical Terms
If your pregnancy is healthy and uncomplicated, the safest time to give birth is between 39 weeks and 40 weeks and 6 days. This is when the baby’s organs are fully mature, NICU admission rates are at their lowest, and the risks associated with postterm pregnancy haven’t yet started to rise. Most babies arrive on their own somewhere in this range, though plenty come a bit earlier or later.
If you’re approaching 37 or 38 weeks and feeling ready to be done, that’s completely understandable. But those last one to two weeks deliver a surprising amount of development, particularly for the brain and lungs. Unless there’s a medical reason to deliver sooner, letting labor begin naturally or waiting until 39 weeks for a planned delivery gives your baby the best start.