Is 37 Weeks Full Term? Early Term vs Full Term

A pregnancy reaching 37 weeks is not considered full term. Since 2013, major medical organizations have classified 37 weeks through 38 weeks and 6 days as “early term,” reserving the label “full term” for births at 39 weeks through 40 weeks and 6 days. The distinction is not just semantic: babies born in those final two weeks of the early-term window face measurably higher rates of breathing problems, intensive care admission, and developmental challenges compared with those who reach 39 weeks.

How the Definition Changed

For decades, any pregnancy between 37 and 42 weeks was lumped together as “term.” Doctors, hospitals, and parents all treated 37 weeks as functionally equivalent to 40 weeks. But accumulating research showed that outcomes varied substantially across that five-week span, particularly for respiratory illness. In late 2012, a joint work group recommended replacing the blanket “term” label with four categories: 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), and postterm (42 weeks and beyond). The American College of Obstetricians and Gynecologists (ACOG) and the Society for Maternal-Fetal Medicine endorsed those designations and encouraged their use by clinicians, researchers, and public health officials.¹

The goal was practical. When everyone called 37 weeks “term,” there was little institutional pushback against scheduling a cesarean or induction at that point for convenience. A more granular vocabulary gave hospitals the language to build policies around the growing evidence that 39 weeks is a meaningful threshold.

Why the Last Two Weeks Matter for Breathing

The lungs are among the last organs to finish maturing. Surfactant, the substance that keeps the tiny air sacs in the lungs from collapsing, ramps up sharply in the final weeks of pregnancy. Animal research has shown that a key surfactant protein first appears in amniotic fluid well before term and rises progressively until birth, and that this protein even plays a role in signaling the start of labor.² In practical terms, this means a baby born at 37 weeks has measurably less surfactant reserve than one born at 39 weeks.

The clinical data reflect this. A large study using target-trial methodology found that the absolute risk of respiratory distress after a planned cesarean at 37 weeks was about 12%, compared with roughly 3% at 39 weeks. For labor inductions, the risk at 37 weeks was around 4%, dropping further by 39 weeks.³ Even outside of cesarean deliveries, respiratory morbidity was consistently highest at 37 weeks and dropped substantially by 38 and again by 39 weeks, regardless of whether labor started spontaneously or was induced.⁴

The route of delivery compounds the effect. Cesarean delivery is associated with more respiratory trouble at every gestational age, but the gap between cesarean and vaginal delivery is largest at 37 weeks and narrows as the pregnancy continues.⁴ That combination of early timing and surgical delivery is what makes elective cesareans before 39 weeks particularly risky for the newborn’s lungs.

Intensive Care Admission and Early Feeding Difficulties

Breathing trouble is the headline risk, but it is not the only reason early-term babies end up in the NICU. Early-term infants also have higher rates of temperature instability, low blood sugar, and jaundice because the liver and metabolic systems are still catching up. In weighted analyses, early-term infants were about 56% more likely to be admitted to the NICU and 50% more likely to never initiate breastfeeding compared with full-term infants.⁵ A secondary analysis of a randomized trial found that babies born after induction at early term were about twice as likely to need a NICU stay of more than four hours compared with those induced at full term.⁶

Breastfeeding challenges are a related but distinct problem. Early-term babies tend to be sleepier and have weaker sucking reflexes, making the first days of feeding harder. Among firstborn infants, about 73% of those born early term were breastfeeding at one month postpartum, compared with roughly 77% of those born at term or later. After adjusting for other factors, early-term newborns were significantly less likely to still be breastfeeding at one month.⁷ The gap is not enormous on an individual level, but across a population it adds up, and for parents counting on breastfeeding, it is worth knowing that a baby born a couple of weeks earlier may need extra support.

Brain Development Does Not Stop at 37 Weeks

The brain grows remarkably fast in the final stretch of pregnancy. Between 35 and 39 weeks, brain volume increases by roughly a third. That growth is not just about size: the cortex is folding, myelination is accelerating, and neural connections are forming that underpin motor control, sensory processing, and early cognition. Research comparing developmental scores at one year of age found that children born at 39 to 41 weeks had measurable advantages over those born at 37 to 38 weeks, leading the authors to flag ongoing brain development during the full-term period as an important medical and policy issue.⁸

These differences are not limited to infancy. A systematic review found that full-term children scored about 5% of a standard deviation higher than early-term children on general cognitive measures, and the gap persisted into adulthood, where full-term cohorts still outperformed early-term cohorts by about 3% of a standard deviation.⁹ Those numbers sound small individually, and for any one child they may be invisible. But at the population level, they shift the curve: more children at the margin needing extra help in school.

Attention, Behavior, and School Performance

Beyond general cognitive ability, early-term birth has been linked to specific neurodevelopmental conditions. A systematic review found that children born early term had up to about 30% increased risk of ADHD, roughly 25% higher risk of language impairments, and anywhere from 8% to 75% greater chance of poorer overall school performance compared with full-term peers.¹⁰

A large U.S. national cohort study put finer numbers on the ADHD link specifically. ADHD prevalence was 5.2% among early-term children compared with 4.5% among those born full term. After adjusting for family and socioeconomic factors, early-term birth was associated with a 10% higher risk of ADHD, a modest but statistically robust finding.¹¹ A Finnish population study echoed this, finding that children born between 36 and 37 weeks had about 25% higher risk of both ADHD and autism spectrum disorder, along with elevated risk of learning disabilities and communication disorders, compared with full-term children.¹²

It is important to keep these numbers in perspective. The absolute risk differences are small. The vast majority of early-term babies develop normally. But for families and policymakers deciding whether to schedule a delivery before 39 weeks without a medical reason, even small shifts in risk across millions of births matter.

Infant Mortality Across the Term Window

The gradient extends to survival as well. A Canadian study found that infant mortality decreased progressively from about 4.6 per 1,000 births at 37 weeks to 1.6 per 1,000 at 41 weeks. Post-neonatal mortality, which captures events like SIDS in the months after birth, was lowest at 40 to 41 weeks. Relative to 41 weeks, infants born at 39 weeks had a 30% higher rate of death from congenital anomalies and roughly 58% higher rate of sudden infant death.¹³ A Scandinavian study similarly found that stillbirth and infant mortality risks were higher among births at 37 and 38 weeks compared with later gestations, with mortality about 20 to 25% lower for births at 40 or 41 weeks.¹⁴

The connection between early-term birth and SIDS risk is not well understood mechanistically, but it aligns with the broader picture of immature brainstem reflexes and arousal mechanisms. The differences are, again, small in absolute terms, but they were part of the evidence that pushed clinicians to rethink the old “term is term” assumption.

Hospital Policies and the 39-Week Rule

The redefinition of term was not just an academic exercise. Hospitals across the United States began implementing “hard-stop” policies that prevented scheduling elective deliveries before 39 weeks without documented medical justification. Early results looked promising. One large hospital system saw NICU admissions drop from about 9.3% to 8.6% of term live births after implementing such guidelines.¹⁵ Another institution reported a significant reduction in short NICU stays specifically.¹⁶

The picture was not uniformly positive, though. Oregon’s statewide hard-stop policy showed no change in NICU admission rates or assisted ventilation, and it came with an unintended consequence: rates of chorioamnionitis, an infection of the membranes surrounding the baby, nearly doubled, likely because some women who would have been delivered were instead waiting longer with ruptured membranes or other evolving situations.¹⁷ This highlights a recurring tension in obstetric policy: blanket rules can help most people while creating new risks for a subset.

When Earlier Delivery Is Medically Necessary

None of the evidence against early-term delivery applies when there is a genuine medical reason to get the baby out. Conditions like preeclampsia, poorly controlled gestational diabetes, placental problems, or fetal growth restriction can make continuing the pregnancy more dangerous than the risks of early delivery. ACOG explicitly states that deferring delivery to 39 weeks is not recommended when a medical or obstetric indication for earlier birth exists.¹⁸

A retrospective study found that chronic maternal conditions like gastrointestinal disease and anemia were associated with significantly higher odds of medically indicated early-term delivery.¹⁹ The decision in these cases is a balancing act: the baby’s lungs and brain would benefit from more time, but the mother’s health or the placenta’s function may not allow it. If your provider recommends delivery at 37 or 38 weeks for a specific complication, that recommendation is based on a calculation that the risks of waiting outweigh the risks of being early.

The ARRIVE Trial and Elective Induction at 39 Weeks

While the evidence discourages elective delivery before 39 weeks, a landmark trial asked a different question: is there any benefit to inducing labor right at 39 weeks rather than waiting for labor to start on its own? The ARRIVE trial randomized over 6,000 low-risk first-time mothers and found that elective induction at 39 weeks did not lead to worse outcomes for the baby and, unexpectedly, resulted in a lower rate of cesarean delivery — about 19% in the induction group versus 22% in the group that waited.²⁰

The trial generated considerable debate. The composite measure of bad neonatal outcomes was not significantly different between the two groups, which means the primary endpoint was technically a null result. Some researchers and midwifery groups pointed out that the trial population was heavily self-selected and may not represent the broader birthing population.²¹ Still, for low-risk first-time mothers who want to discuss their options at 39 weeks, the trial provides evidence that induction at that specific point does not carry the same risks as induction at 37 or 38 weeks. The emphasis on 39 weeks is critical — the same logic does not extend two weeks earlier.

What Elective Early Cesarean Means for the Baby

It is worth separating elective induction from elective cesarean in the early-term window, because the data tell different stories. When researchers compared elective inductions at 37 to 38 weeks with expectant management through 39 to 40 weeks, the induced babies did not have significantly higher rates of respiratory problems, sepsis, or NICU admission, though they did have slightly higher odds of feeding difficulty. Elective cesareans at the same early-term gestational age, however, were a different matter entirely, with 13% to 66% increased odds of various adverse outcomes.²² The likely explanation is that labor itself helps clear fluid from the lungs and triggers hormonal cascades that prepare the baby for breathing. A cesarean performed before labor begins skips that preparation.

Long-Term Health Beyond Childhood

Most of the attention around early-term birth focuses on the newborn period and school years, but a growing body of research suggests the effects may extend further. A recent review noted that late-preterm and early-term birth has been linked to increased susceptibility to neurodevelopmental delays, behavioral and neuropsychiatric problems, and even chronic diseases into adulthood.²³ The mechanisms behind such long-range effects are not fully mapped, but they likely involve the downstream consequences of subtle differences in organ maturation, metabolic programming, and early-life stress on developing systems.

What Happens When Pregnancy Goes Too Long

If early-term birth carries risks, it is natural to wonder whether longer is always better. It is not. The four-category system introduced in 2013 also distinguishes late term (41 weeks) from postterm (42 weeks and beyond) for good reason. After 41 weeks, the placenta begins to show signs of aging: reduced growth, impaired transport of nutrients, increased oxidative stress, and cellular changes that resemble the damage seen in conditions like preeclampsia and fetal growth restriction.²⁴ Prolonged pregnancy is associated with increasing perinatal mortality, which is why most providers will recommend induction by 41 or 42 weeks even in otherwise uncomplicated pregnancies.

The sweet spot, as far as the data show, is 39 to 40 weeks. That window balances completed organ maturation with a placenta that is still functioning well. This does not mean every baby born at 37 weeks will have problems or that every pregnancy going to 41 weeks is risk-free. It means that when clinicians and families have a choice about timing, aiming for that full-term window gives the baby the best statistical shot.

Why Humans Are Born Comparatively Immature

From an evolutionary standpoint, human infants arrive in the world remarkably helpless compared with other primates. The classic explanation, sometimes called the obstetrical dilemma, proposes that the human pelvis had to stay narrow enough for bipedal walking while the human brain grew dramatically larger. The suggested compromise was to give birth earlier in neurological development, producing babies that are physically and neurologically immature relative to other species.²⁵ Whether or not that hypothesis captures the full picture — and it has been debated and refined for decades — it underscores something relevant to the early-term question: human babies are already born at a relatively early stage of brain development. Shaving off even two additional weeks of that development pushes them further along a continuum of immaturity that evolution did not design much margin into.