Term definitions checked against ACOG Committee Opinion No. 579.
Full term is 39 weeks 0 days through 40 weeks 6 days. Early term, 37 weeks 0 days through 38 weeks 6 days, is a different category, not a synonym. The distinction is not academic hair-splitting; it exists because outcome data showed babies born in the earlier window need more breathing and feeding support, on average, than babies born a week or two later.
ACOG's four term categories
| Category | Weeks of gestation |
|---|---|
| Preterm | Before 37 weeks 0 days |
| Early term | 37 weeks 0 days to 38 weeks 6 days |
| Full term | 39 weeks 0 days to 40 weeks 6 days |
| Late term | 41 weeks 0 days to 41 weeks 6 days |
| Post-term | 42 weeks 0 days and beyond |
These categories come from ACOG Committee Opinion No. 579, published in 2013 by a work group convened specifically to replace the older, looser "term" label with something more precise.
Why 37 weeks stopped counting as full term
Before 2013, anything from 37 to 42 weeks was simply called "term," with no distinction inside that range. The work group's review found that neonatal outcomes, particularly respiratory problems, differed measurably between babies born at 37 to 38 weeks and those born at 39 to 40 weeks, with the lowest rate of complications at 39 to 40 weeks specifically. That evidence is why early term now stands apart from full term rather than being folded into it.
What changes in the final weeks
Between 37 and 40 weeks, brain volume keeps increasing, the lungs continue maturing toward independent breathing, and the liver builds the nutrient reserves a newborn draws on before regular feeding is established. None of this happens on a switch at 37 weeks; it is why the final stretch of a full-term pregnancy is doing real developmental work rather than just waiting.
Going past 40 weeks
Passing 40 weeks is common and not automatically a concern on its own. Providers typically step up monitoring of the placenta and fetal wellbeing once a pregnancy runs into the late-term window, and most discuss induction options somewhere between 41 and 42 weeks, since post-term risk factors, including placental changes, become more relevant past that point. Preterm birth, before 37 weeks, is tracked separately by the CDC and carries its own set of concerns unrelated to the term categories above.
Early delivery for a medical reason
None of this means an early-term or preterm delivery is automatically a problem; sometimes delivering before 39 weeks is the safer option for a specific medical reason, decided with your provider. The categories above describe population-level patterns, not a rule for any individual birth. See the Due Date Calculator to check exactly where your own pregnancy sits against these ranges today.
Find your current week
See how far along you are and where you fall against these term categories.