A typical pregnancy lasts about 40 weeks, counted from the first day of your last menstrual period. That works out to roughly 280 days, or just over nine calendar months. But the actual time your baby is growing inside you is closer to 38 weeks, because the standard counting method starts about two weeks before conception occurs.
Only about 5% of women give birth on their exact due date. Most babies arrive somewhere between 37 and 41 weeks, and that wide window is completely normal.
Why the Math Feels Confusing
Pregnancy timelines can be disorienting because doctors count from a date you weren’t actually pregnant yet. Gestational age starts on the first day of your last period, which is typically about two weeks before the egg is fertilized. So when your doctor says you’re “four weeks pregnant,” the embryo has only existed for roughly two weeks.
This convention exists because most women know when their last period started but don’t know the exact day they conceived. A study tracking women from ovulation found that the median time from ovulation to birth was 268 days, or 38 weeks and 2 days. Add two weeks to account for the pre-ovulation phase and you land right around that familiar 280-day figure.
How Due Dates Are Calculated
Your estimated due date is set at 280 days after the first day of your last period. This assumes a regular 28-day cycle with ovulation happening on day 14. If your cycles are longer, shorter, or irregular, this estimate can be off by days or even weeks. That’s one reason a first-trimester ultrasound is often used to refine or replace the period-based estimate, since early fetal measurements are more consistent across pregnancies.
Even with the best dating methods, a due date is a rough target. In a study of over 1,000 women who went into labor naturally, 80% delivered within a 19-day window (between 261 and 280 days). The average was 272 days, about a week before the standard due date.
What Counts as “Full Term”
Not all weeks near the due date are considered equal. Medical guidelines break the final stretch into 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
These distinctions matter because babies born at 39 weeks generally have better outcomes than those born at 37, even though both fall within the broad “term” range. The brain, lungs, and liver are still maturing during those final weeks.
Factors That Shorten or Lengthen Pregnancy
Several things influence whether you’ll deliver closer to 37 weeks or 41. One of the most consistent factors is whether you’ve given birth before. First-time mothers tend to carry slightly longer than women in subsequent pregnancies.
Pre-pregnancy weight also plays a role. Research comparing women across BMI categories found that those with a higher BMI in the first trimester carried longer on average. Women with a BMI under 19.8 delivered at an average of 37.8 weeks, while women with a BMI over 29 averaged 39.6 weeks. The relationship was statistically significant, and higher BMI may increase the chance of going past 40 weeks.
Natural variation between individuals is surprisingly large. Even after researchers excluded preterm births and medical complications, the standard deviation was about 9 days. That means two healthy pregnancies with no complications can differ by nearly three weeks and both be perfectly normal.
Carrying Twins or Multiples
If you’re carrying more than one baby, expect a shorter pregnancy. A large population study found the average duration for twin pregnancies was 37.0 weeks, compared to 39.6 weeks for singletons. That’s roughly two and a half weeks earlier. The optimal window for twin delivery, where outcomes are best, falls between 37 and 39 weeks rather than the 39 to 41 weeks typical for a single baby.
Triplets and higher-order multiples arrive even earlier on average, often around 33 to 34 weeks.
What Happens When Pregnancy Goes Past 42 Weeks
Going a few days past your due date is common and usually fine. But pregnancies that extend beyond 42 weeks carry increased risks. The placenta begins to function less efficiently, which can reduce the flow of oxygen and nutrients to the baby. Amniotic fluid levels may drop, increasing the chance of umbilical cord compression.
Specific risks of post-term pregnancy include stillbirth, the baby growing unusually large (which complicates delivery), the baby inhaling meconium into the lungs, and decreased amniotic fluid. For the mother, there’s a higher chance of needing an assisted delivery or cesarean section, along with increased risk of infection and heavy bleeding after birth. This is why most providers will discuss induction if you haven’t gone into labor by 41 to 42 weeks.
Why Human Pregnancy Lasts Nine Months
Compared to our closest relatives, human pregnancy is relatively long. Chimpanzees carry for about 32 weeks, and gorillas and orangutans for 37 to 38 weeks. Human gestation is 37 days longer than you’d predict for a primate of our body size.
One longstanding explanation is that human babies need to be born before their heads grow too large to fit through the birth canal, which narrowed as our ancestors evolved to walk upright. If human newborns were born at the same neurological stage as baby chimpanzees, pregnancy would need to last 18 to 21 months. Instead, our babies are born relatively helpless and do most of their brain development outside the womb. More recent research suggests that the metabolic cost of pregnancy also plays a role: by about 40 weeks, the energy demands of the growing fetus approach the maximum a mother’s body can sustain, and that limit may be what triggers the timing of birth as much as pelvic size does.