A human pregnancy lasts about 40 weeks, or 280 days, counted from the first day of the last menstrual period. That’s the standard number you’ll see on pregnancy apps and hear from your doctor. But the actual time from conception to birth is shorter, averaging 38 weeks and 1 day, because the counting method includes roughly two weeks before ovulation and fertilization even happen.
Why Pregnancy Is Counted From Your Last Period
The 40-week figure uses a formula dating back to the 1800s: take the first day of your last menstrual period, add 280 days, and you get an estimated due date. This works reasonably well for someone with a regular 28-day cycle who ovulates around day 14. The problem is that many people don’t fit that template. Cycles vary, ovulation timing varies, and roughly half of women don’t accurately recall the exact date of their last period.
That’s why a first-trimester ultrasound is now considered the most accurate way to date a pregnancy. Between 6 and 14 weeks, measuring the embryo from head to rump can pin down gestational age within 5 to 7 days. In one study, 40% of women who received a first-trimester ultrasound had their due date adjusted by more than 5 days compared to period-based dating. If the ultrasound and period-based dates differ by more than a week, providers will typically go with the ultrasound.
What “Full Term” Actually Means
Pregnancy used to be considered “term” at any point from 37 to 42 weeks. That changed in 2013, when the American College of Obstetricians and Gynecologists replaced that broad label with four more precise 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
The distinction matters because babies born at 37 or 38 weeks, while generally healthy, have slightly higher rates of breathing problems and feeding difficulties compared to those born at 39 or 40 weeks. The 39-week mark is when the lungs, brain, and liver have had enough time to fully mature.
How Much Natural Variation Is Normal
Even among perfectly healthy pregnancies with no complications, the range of when labor starts naturally is surprisingly wide. A study published in Human Reproduction tracked pregnancies from the exact day of ovulation (removing the guesswork of period-based dating) and found that the time from ovulation to birth averaged 268 days, with a standard deviation of 10 days. That means most healthy pregnancies fell somewhere between 258 and 278 days from ovulation, a spread of nearly three weeks.
Several factors push the number in one direction or the other. Older maternal age, higher body weight, and being pregnant for the first time are all associated with slightly longer gestations. But even accounting for these factors, a good deal of the variation remains unexplained. Your body simply has its own timeline, and a due date is better understood as the middle of a window than a target to hit.
How Few Babies Arrive on Their Due Date
The due date printed on your chart is an estimate, not a prediction. Because inductions and scheduled deliveries are now so common in the U.S. and other Western countries, it’s difficult to know exactly what percentage of people would naturally deliver on their due date. But the data consistently shows that the vast majority of babies arrive sometime in the weeks around that date rather than on it. Treating the due date as a rough center point, with a realistic window of 37 to 42 weeks, is a more accurate way to think about timing.
What Happens When Pregnancy Goes Past 41 Weeks
Most pregnancies that stretch past the due date are still perfectly safe, but the risks do increase gradually. The risk of stillbirth rises from about 2.1 per 10,000 ongoing pregnancies at 37 weeks to 6.1 per 10,000 at 41 weeks and 10.8 per 10,000 at 42 weeks. In absolute terms, these numbers are still very small, but the trend is clear enough that guidelines address it directly.
The World Health Organization recommends offering induction at 41 weeks but not before. ACOG recommends induction between 42 and 43 weeks based on strong evidence, and considers induction between 41 and 42 weeks reasonable based on more limited evidence. Canadian guidelines recommend routine induction sometime during the 41st week. The UK’s NICE guidelines emphasize giving every woman with an uncomplicated pregnancy the opportunity to go into labor on her own, while explaining that certain risks begin to increase after 41 weeks.
For first-time mothers at low risk, some providers also offer the option of elective induction at 39 weeks, based on a large trial (known as ARRIVE) that found comparable or slightly improved outcomes. This remains a personal decision rather than a blanket recommendation, and depends on individual circumstances and preferences.
The Short Answer
Pregnancy lasts about 40 weeks from the last menstrual period, or about 38 weeks from conception. Healthy, uncomplicated pregnancies naturally vary by as much as three weeks in either direction. The window considered full term is 39 weeks through 40 weeks and 6 days, though babies born anywhere from 37 to 41 weeks generally do well. Your due date is the best guess at the midpoint of that range, not a deadline.