The average pregnancy lasts about 40 weeks when counted from the first day of your last menstrual period, which is the standard method used in clinical care. But when researchers tracked pregnancies from the actual date of ovulation, the median length was 268 days, or 38 weeks and 2 days. That two-week gap exists because the conventional 40-week count includes roughly two weeks before conception even occurs.
What surprises most people is how much healthy pregnancies can vary. Even among uncomplicated pregnancies where the exact date of conception was known, the range spanned 37 days from shortest to longest. Your due date is an estimate, not a deadline.
Why the 40-Week Number Exists
Pregnancy is conventionally dated from the first day of your last menstrual period (LMP), not from conception. Since ovulation typically happens about two weeks into a cycle, this method adds roughly 14 days to the biological length of pregnancy. It became the standard because most people can recall when their period started, even if they don’t know when they ovulated.
The problem is that only about half of women accurately recall their LMP date, and cycle lengths vary. That’s why ultrasound has become the preferred way to confirm or adjust a due date. A first-trimester ultrasound (before 14 weeks) is the most accurate dating method available. In one study, 40% of women who received a first-trimester ultrasound had their due date shifted by more than 5 days compared to LMP-based dating. A pregnancy without an ultrasound before 22 weeks is considered suboptimally dated by the American College of Obstetricians and Gynecologists.
What “Full Term” Actually Means
Not all weeks near the due date are equal. ACOG breaks the term period into distinct categories:
- Early term: 37 weeks through 38 weeks, 6 days
- Full term: 39 weeks through 40 weeks, 6 days
- Late term: 41 weeks through 41 weeks, 6 days
- Post-term: 42 weeks and beyond
These distinctions matter because babies born even a week or two earlier within the “term” window have slightly different outcomes than those born at 39 or 40 weeks. The full-term window of 39 to 40 weeks is when babies are best prepared for life outside the womb, with more mature lungs, brain, and liver function.
First Pregnancies Tend to Run Longer
If this is your first baby, expect to go a bit longer. A large analysis of electronic health records found that first-time mothers deliver at an average of 275.9 days, compared to 274.5 days for women who have given birth before. That’s roughly a day and a half difference. It’s statistically real but modest, and it’s an average: plenty of first-time mothers deliver before their due date, and plenty of experienced mothers go past theirs.
Maternal age also plays a role. For each additional year of age, the risk of preterm delivery (before 37 weeks) increases slightly. The effect is small on an individual level but becomes meaningful across populations, particularly for mothers in their late 30s and 40s.
Why Pregnancies Vary So Much
Even in the most carefully tracked pregnancies, natural variation is significant. A 2013 study published in Human Reproduction pinpointed ovulation dates using hormone tracking and still found a five-week spread in delivery timing among healthy, uncomplicated pregnancies. Some of this variation correlated with events at the very start of pregnancy: embryos that took longer to implant in the uterine wall tended to result in longer gestations overall.
When pregnancy length is calculated from LMP, the variation is even wider, with a coefficient of variation of 4.9% compared to 3.7% when measured from ovulation. The extra scatter comes from differences in when women ovulate relative to their period. Two women with the same LMP date could have conceived days apart and therefore have genuinely different due dates, even though their charts look identical.
What Happens When Pregnancy Goes Past 41 Weeks
Most pregnancies that stretch past the due date are still perfectly fine, but risks do increase as weeks tick by. The risk of stillbirth rises from 0.11 per 1,000 pregnancies at 37 weeks to 3.18 per 1,000 at 42 weeks. Between 40 and 41 weeks specifically, the risk of stillbirth increases by 64%. Newborn mortality also jumps for babies born at 42 weeks compared to 41 weeks.
These numbers are still small in absolute terms, but they explain why most providers recommend increased monitoring after 41 weeks and typically discuss induction before 42 weeks. For post-term pregnancies, the concern isn’t just stillbirth: the placenta gradually becomes less efficient at delivering oxygen and nutrients, and larger babies can mean more difficult deliveries.
Elective Induction at 39 Weeks
A shift in practice over the past several years has made 39-week induction a reasonable option for some first-time mothers. Research found that healthy women carrying a single baby who were induced at 39 weeks had lower rates of cesarean delivery compared to those who waited for labor to start on its own. They also had lower rates of preeclampsia and gestational hypertension.
This doesn’t mean every pregnancy should be induced at 39 weeks. The option applies specifically to first-time mothers with uncomplicated, single pregnancies who are otherwise healthy. For women who have given birth before, or who have specific medical conditions, the calculus is different. The key takeaway is that reaching 40 weeks is not a medical requirement for a healthy delivery, and going past 41 weeks warrants a conversation about next steps.