A pregnancy is considered full term from 39 weeks and 0 days through 40 weeks and 6 days of gestation. That window — 39w0d to 40w6d — is the American College of Obstetricians and Gynecologists (ACOG) definition, set in 2013, and it replaced the older idea that anything from 37 weeks onward counted as "term." The change matters: a baby born at 37 weeks and a baby born at 40 weeks are both past the preterm line, but their lungs, brains, and feeding skills are at noticeably different stages, and the last three weeks of pregnancy do real developmental work.
The confusion is understandable, because "term" is used in two ways. In everyday speech, term often means "not premature," and anything from 37 weeks qualifies. In clinical use, term is now split into four bands, and only one of them is called full term. Knowing which band a pregnancy is in explains why a clinician may be relaxed about a birth at 39 weeks, cautious about an elective delivery at 37 weeks, and watchful once 41 weeks passes.
The four term bands: early, full, late, and postterm
The four term bands divide every pregnancy from 37 weeks onward, and each band carries a different level of concern. Early term runs from 37 weeks and 0 days to 38 weeks and 6 days. Full term runs from 39 weeks and 0 days to 40 weeks and 6 days. Late term runs from 41 weeks and 0 days to 41 weeks and 6 days. Postterm runs from 42 weeks and 0 days onward. A due date, by definition, falls at 40 weeks and 0 days — right at the start of full term, not at its end.
| Band | Gestational age | What it means |
|---|---|---|
| Early term | 37w0d – 38w6d | Not preterm, but the baby is still maturing |
| Full term | 39w0d – 40w6d | The ideal window for birth |
| Late term | 41w0d – 41w6d | Past the due date, under closer watch |
| Postterm | 42w0d and beyond | Higher risk; delivery usually planned |
The reason full term starts at 39 weeks is evidence, not tidiness. Babies born at 37 to 38 weeks — early term — are more likely than babies born at 39 to 40 weeks to need help with breathing, feeding, and temperature control, and to spend time in a neonatal unit. A large body of research reviewed by ACOG before the 2013 redefinition found that respiratory problems, low blood sugar, and neonatal intensive care admissions all fall as gestational age rises from 37 to 39 weeks, then level off. That is why elective deliveries — inductions or planned caesareans with no medical reason — are not recommended before 39 weeks: the baby gains measurable maturity in those final two weeks, and there is no benefit to giving it up without a reason.
What is happening in those final weeks explains the numbers. The baby's brain grows rapidly in the last weeks of pregnancy, adding connections that support breathing regulation, feeding coordination, and temperature control. The lungs finish producing surfactant, the substance that keeps air sacs open, and the liver builds up glycogen stores that steady blood sugar after birth. Weight gain runs at roughly half a pound (about 225 grams) per week in the final weeks, and much of that is the fat layer that helps the baby stay warm outside the womb. A baby born at 37 weeks has done most of this work, but not all of it — which is exactly why the early-term band exists as a warning, not a reassurance.
Why the due date sits inside full term
The due date sits at 40 weeks and 0 days, which is the first day of the second week of full term, and that placement is deliberate. Pregnancy is counted as 40 weeks from the first day of the last menstrual period, using Naegele's rule, and the due date is an estimate at the centre of a window, not an appointment the baby is expected to keep. Only about 5% of babies are born on their exact due date, while roughly 80% are born within two weeks either side of it — so a birth anywhere in the full-term window, from 39w0d to 40w6d, is entirely on schedule.
That window framing changes how the final weeks feel. A pregnancy at 39 weeks is not "a week early": it is in the ideal band. A pregnancy at 41 weeks is not yet postterm: it is late term, and the usual response is closer monitoring — non-stress tests and fluid checks, typically twice a week — rather than immediate intervention. If you are tracking where you are, the how many weeks pregnant calculator gives your current week from your last period, and the pregnancy week-by-week guide describes what is developing at each stage. The due date calculator works out your 40-week date from your last period, and if your dates came from a scan instead, the ultrasound dating is usually the more accurate anchor, since ovulation does not always fall on day 14 of the cycle. The same counting confusion shows up in months: our guide to how many weeks 5 months pregnant is explains why month 5 spans weeks 18 to 22 rather than a single week.
What happens before and after full term
Before full term, the bands run downward from early term into preterm territory. Late preterm covers 34w0d to 36w6d, moderately preterm covers 32w0d to 33w6d, very preterm covers 28w0d to 31w6d, and extremely preterm covers anything before 28 weeks. Each step down carries higher risks than the one above it, which is why clinicians work hard to delay a threatened preterm birth even by a few days: at 34 weeks, a course of corticosteroids to speed lung maturity takes about 48 hours to reach full effect, and those 48 hours are often the whole point of the treatment.
After full term, the picture changes in the other direction. In late term, from 41w0d, the placenta — the organ feeding the baby — begins to age, and the fluid around the baby can fall. The risk of stillbirth, while still low in absolute terms, rises gradually after 41 weeks, roughly doubling from about 1 in 1,000 at 40 weeks to about 2 in 1,000 at 42 weeks in large population studies. That is why most clinicians discuss induction between 41 weeks and 42 weeks rather than waiting indefinitely: the ARRIVE trial, published in 2018, found that induction at 39 weeks in first-time pregnancies with no complications did not raise the caesarean rate and slightly reduced some complications, but routine induction at 39 weeks is still not standard practice everywhere, and the decision is made with the individual pregnancy, not the calendar alone. A pregnancy that reaches 42 weeks is postterm, and delivery is usually planned promptly at that point, since the risks of waiting begin to outweigh the benefits of more time.
One useful comparison is with a medically indicated early delivery. When a condition such as pre-eclampsia or cholestasis makes waiting riskier than delivering, clinicians do deliver before 39 weeks — sometimes well before it. Our guide to whether delivery by 37 weeks is necessary for cholestasis of pregnancy shows how that trade-off is weighed condition by condition: the baby's maturity is one side of the scale, and the mother's or baby's safety is the other. The 39-week rule is about elective deliveries with no medical reason, not about deliveries where a condition forces the decision.
How gestational age is counted — and why it can be off
Gestational age is counted from the first day of the last menstrual period, not from conception, which adds about two weeks to the baby's true age. Conception usually happens around ovulation, roughly two weeks after the period starts, so a pregnancy described as 8 weeks pregnant has an embryo about 6 weeks old. This convention is universal in obstetrics, and it is why a "40-week pregnancy" is really about 38 weeks from conception.
The count can be off when periods are irregular, when ovulation falls much earlier or later than day 14, or when the last period is misremembered — and an error here moves the whole term-band calculation. That is why an early ultrasound, ideally between 8 and 13 weeks, is the most reliable dating method: it measures the baby directly, and if it disagrees with the period-based date by more than a few days, the scan date usually replaces it. Later scans are less accurate for dating, because babies vary more in size as pregnancy advances, so a third-trimester scan cannot reliably re-date a pregnancy. If your due date was revised after an early scan, trust the revised date: it is the one your term band, your screening windows, and your induction discussions are all based on.
When to call your clinician in the final weeks
Full term is the ideal window, but it is not a guarantee that everything will be straightforward, and knowing the warning signs matters as much as knowing the bands. In the final weeks, monitoring often steps up: a non-stress test in pregnancy checks the baby's heart rate against its movements, and it becomes a routine part of late-term and postterm care. Between appointments, the signs that warrant a prompt call are reduced fetal movement from the baby's usual pattern, vaginal bleeding, leaking fluid, a severe headache, visual changes such as blurring or flashing lights, sudden swelling of the face or hands, or a fever above 38°C (100.4°F). Reduced movement is the one people most often wait on, and it is the one least worth waiting on: a change in the baby's usual pattern is always a reason to call the same day, at any week of pregnancy.
The bottom line is simpler than the bands make it sound. Full term means 39 weeks and 0 days to 40 weeks and 6 days, and it is the window every pregnancy is aiming for. Early term is close but not quite there, late term is past the due date but not yet a problem, and postterm is where delivery is usually planned. If you are counting down, you can work out your own dates with the due date calculator and check which band you are in with the how many weeks pregnant calculator — and remember that the due date is the middle of the window, not the deadline.