A pregnancy lasts 280 days, or 40 weeks, counted from the first day of your last menstrual period. That’s roughly nine months and one week. But this number is an estimate, not a guarantee. Only about 5% of babies arrive on their exact due date.
How the 280-Day Count Works
The 280-day figure starts from a date that might seem odd: the first day of your last period, not the day you actually conceived. Conception typically happens about two weeks later, around ovulation. So while the calendar says 40 weeks, the actual time a baby spends developing is closer to 266 days, or 38 weeks.
Providers use this counting method because most people can recall when their period started more easily than when they ovulated. The system dates back to a formula called Naegele’s rule: take the first day of your last period, add seven days, and count forward nine months. It assumes a 28-day menstrual cycle with ovulation on day 14, which doesn’t hold true for everyone. If your cycles are shorter, longer, or irregular, the estimate can be off by days or even weeks.
How Your Due Date Gets Refined
Remembering the exact start of your last period is harder than it sounds. Research from the American College of Obstetricians and Gynecologists shows that only about half of women accurately recall the date. That’s one reason early ultrasound has become the preferred tool for pinning down a due date.
A first-trimester ultrasound (before 14 weeks) measures the embryo from head to rump and can estimate gestational age within a window of plus or minus 5 to 7 days. In one study, 40% of women who received a first-trimester ultrasound had their due date adjusted because it differed from the period-based estimate by more than five days. By contrast, only 10% of women who waited until the second trimester for their first scan needed an adjustment, suggesting that getting an early ultrasound catches dating errors that would otherwise go unnoticed.
What “Full Term” Actually Means
Not every delivery at or near 40 weeks carries the same label. The National Institutes of Health and major obstetric organizations define four categories for the final stretch of pregnancy:
- 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 categories exist because even a week or two makes a difference in newborn health. Babies born in the full-term window (39 to 40 weeks) tend to have better outcomes than those born at 37 weeks, even though 37 weeks was once considered “term” without any qualifier. The distinction matters for decisions about scheduling inductions or cesarean deliveries.
Why Few Pregnancies Hit 280 Days Exactly
The 280-day figure is an average, and individual pregnancies vary widely. About 95 out of 100 babies are born on a day that is not their due date. Some of this variation comes down to biology that no calendar formula can capture: the timing of ovulation, how quickly the embryo implants, and the unique hormonal signals between the placenta and the uterus.
Whether this is your first or second pregnancy also plays a role. First pregnancies tend to run slightly shorter on average, roughly a third of a day less than second pregnancies, according to a large analysis of gestational duration and parity. First pregnancies also show more variability, meaning they’re less predictable in either direction. If you’ve had a previous preterm delivery, that history interacts with the number of pregnancies you’ve had, shifting the expected length by as much as 4 to 5 days.
Counting the Days Yourself
If you want a rough due date before your first prenatal visit, start with the first day of your last period and add 280 days (or 40 weeks). Most phone calendars and free online calculators can do this in seconds. Keep in mind that this is a starting point. Your provider will likely confirm or adjust it with an ultrasound during the first trimester.
A practical way to think about it: your due date marks the middle of a window, not a deadline. Most healthy pregnancies end somewhere between 39 and 41 weeks, giving you a realistic range of about 273 to 293 days. Planning for that two-week spread, rather than circling one specific date, tends to set more realistic expectations for when your baby will actually arrive.