A normal pregnancy lasts 40 weeks, or about 280 days, counted from the first day of your last menstrual period. That said, healthy babies routinely arrive anywhere from 37 to 41 weeks, and the medical definition of “full term” is narrower than most people realize: 39 weeks through 40 weeks and 6 days.
Why Pregnancy Is Counted as 40 Weeks
The 40-week number comes from a formula called Naegele’s Rule, which has been used for over a century. It starts the clock on the first day of your last period, not the day you actually conceived. Since ovulation typically happens about two weeks into a menstrual cycle, your baby’s biological age is roughly two weeks less than your “gestational age” at any point in pregnancy. The formula assumes a 28-day cycle with ovulation on day 14, so if your cycles are longer or shorter, your true due date may shift by several days.
To estimate your due date using this method: take the first day of your last period, count back three calendar months, then add one year and seven days. That landing date is 40 weeks out, and it’s the number your provider will use as a starting point.
How Accurate Due Dates Really Are
Only about half of women accurately recall the start date of their last period, which makes the 40-week calculation unreliable on its own. That’s why a first-trimester ultrasound (up to about 14 weeks) is considered the most accurate way to date a pregnancy. By measuring the embryo’s length from crown to rump, providers can pin down gestational age to within five to seven days.
The difference matters more than you might expect. 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. Among women who didn’t get an ultrasound until the second trimester, only 10% needed an adjustment. Earlier imaging catches dating errors that later scans miss.
The Four Categories of Term Pregnancy
Doctors no longer treat everything from 37 to 42 weeks as one big bucket. In 2013, the American College of Obstetricians and Gynecologists and the National Institutes of Health introduced four distinct categories:
- Early term: 37 weeks through 38 weeks and 6 days
- Full term: 39 weeks through 40 weeks and 6 days
- Late term: 41 weeks through 41 weeks and 6 days
- Post-term: 42 weeks and beyond
The distinction exists because outcomes differ across these windows. Babies born at 37 or 38 weeks are more likely to face breathing difficulties, feeding problems, and temperature regulation issues than those born at 39 or 40 weeks. The differences for any individual baby tend to be small, but across large populations they show up consistently in health, development, and even educational outcomes later in life. This is why providers generally avoid scheduling elective deliveries before 39 weeks unless there’s a medical reason.
What Affects How Long Your Pregnancy Lasts
No two pregnancies follow the same timeline, even in the same person. First-time mothers tend to carry slightly longer, with an average gestational length of about 275.9 days (39 weeks and 3 days), compared to 274.5 days for women who have given birth before. That gap of roughly a day and a half is modest, but it’s consistent across large datasets.
Genetics play a meaningful role. Research estimates that fetal genes account for 11 to 35 percent of the variation in pregnancy length, while maternal genetics contribute another 13 to 20 percent. Environmental factors, including nutrition, stress, altitude, and physical activity, account for an even larger share of the variation than genetics alone. This is why your pregnancy may not match your mother’s or your sister’s, even though you share much of the same DNA.
The Three Trimesters
Pregnancy is divided into three roughly equal stages:
- First trimester (weeks 1 to 12): Organs form, the heart begins beating, and the embryo grows from a cluster of cells to about 3 inches long.
- Second trimester (weeks 13 to 27): The baby begins to move noticeably, fingerprints develop, and the lungs start maturing.
- Third trimester (weeks 28 to 40): Rapid weight gain, brain development, and positioning for birth.
Most of the growth in weight happens in the third trimester, which is why those final weeks matter so much for a baby’s readiness at birth.
What Happens If Pregnancy Goes Past 41 Weeks
About 10 percent of pregnancies extend beyond 41 weeks. When that happens, providers typically begin fetal monitoring to check the baby’s heart rate and amniotic fluid levels. The concern is that the placenta gradually becomes less efficient at delivering oxygen and nutrients as it ages.
The risk of stillbirth or infant death roughly doubles in pregnancies that continue past 42 weeks: approximately 4 to 7 per 1,000 deliveries, compared to 2 to 3 per 1,000 for pregnancies between 37 and 42 weeks. Because of this, induction of labor is commonly offered between 41 and 42 weeks. Many providers will schedule an induction once you reach 41 weeks, though the exact timing depends on your health, your baby’s condition, and your preferences.
Going a few days past your due date is completely normal. Going a full two weeks past it is where the medical calculus shifts and closer monitoring becomes standard.