A standard pregnancy lasts 280 days, or 40 weeks, counted from the first day of your last menstrual period. Counted from the actual date of conception, the number drops to about 266 days. Both figures are averages, and healthy pregnancies routinely fall on either side of them.
Why There Are Two Different Numbers
The 280-day figure exists because most people don’t know exactly when conception happened, but they do know when their last period started. Since ovulation typically occurs about 14 days into a 28-day cycle, doctors add those two weeks to the front of the pregnancy. That’s why you’re already considered “two weeks pregnant” on the day you conceive.
The formula most providers use, called Naegele’s Rule, works like this: take the first day of your last menstrual period, count back three calendar months, then add one year and seven days. It assumes a 28-day cycle, which not everyone has. If your cycles are longer or shorter, your actual due date may shift by several days in either direction.
When conception is known precisely, as it is with IVF, the estimated due date is calculated by adding 266 days to the conception date. That 266-day number reflects the true biological length of pregnancy, while the 280-day number is a clinical convenience built around the menstrual calendar.
How Much Pregnancy Length Actually Varies
Due dates are estimates, not deadlines. A study published in Human Reproduction tracked pregnancies where the exact date of ovulation was confirmed. Even after excluding preterm births and complications, the range of pregnancy lengths spanned 37 days. The median time from ovulation to birth was 268 days (38 weeks and 2 days), not the textbook 266.
Part of this variation comes from the way pregnancy is measured. When calculated from the last menstrual period, the spread is wider (a coefficient of variation of 4.9%) than when calculated from ovulation (3.7%), simply because not every person ovulates on day 14. Several factors influence where your pregnancy falls within that range. Older maternal age and first pregnancies are both associated with longer gestation. Higher and lower body mass index can push the timeline in either direction, with obesity linked to a greater chance of going past the due date.
Clinical Categories for Term Pregnancy
Not all “term” pregnancies are treated the same. In 2013, medical organizations replaced the single label of “term” with four more specific categories:
- Early term: 37 weeks through 38 weeks and 6 days (259 to 272 days)
- Full term: 39 weeks through 40 weeks and 6 days (273 to 286 days)
- Late term: 41 weeks through 41 weeks and 6 days (287 to 293 days)
- Post-term: 42 weeks and beyond (294+ days)
These distinctions matter because the last few weeks of pregnancy involve critical development. The third trimester is when fetal organs mature for life outside the womb. Lungs, for instance, produce surfactant, a substance essential for breathing air, during this final stretch. Babies born at 37 weeks are far more likely to need breathing support than those born at 39, which is why the “full term” label now starts at 39 weeks rather than 37.
The Upper Limit
Most experts recommend inducing labor between 41 and 42 weeks if it hasn’t started on its own. Beyond 42 weeks (294 days), the placenta gradually becomes less efficient at delivering oxygen and nutrients, and the risks of complications rise. Your provider will typically start monitoring more closely once you pass your due date, with the specific timing depending on your health and pregnancy history.
So while the short answer is 280 days from your last period or 266 from conception, a pregnancy that lasts anywhere from about 259 to 293 days still falls within the broad range of normal. Your actual number will depend on your cycle length, your body, and when labor decides to start.