A human pregnancy lasts 40 weeks, or 280 days, counted from the first day of the last menstrual period (LMP). That’s roughly nine calendar months. But the actual time from conception to birth is shorter, closer to 38 weeks, because the clock starts about two weeks before ovulation and fertilization occur.
Why the Count Starts Before Conception
Pregnancy is dated from the first day of your last menstrual period, not from the day you conceived. That convention exists because most people can recall when their period started but not when they ovulated. Since ovulation typically happens around day 14 of a 28-day cycle, roughly two weeks of “pregnancy” pass before an egg is even fertilized.
When researchers tracked pregnancies where the exact date of ovulation was known, the median time from ovulation to birth was 268 days, or 38 weeks and 2 days. The 280-day figure used in clinical practice simply adds those two pre-conception weeks back in.
How Your Due Date Is Calculated
The traditional method, known as Naegele’s rule, adds 280 days to the first day of your LMP. It assumes a regular 28-day cycle with ovulation on day 14. In reality, cycle lengths vary, ovulation timing shifts, and roughly half of women don’t accurately recall their LMP date. All of that introduces error.
A first-trimester ultrasound is the most accurate way to confirm a due date. Measuring the embryo’s crown-to-rump length before 14 weeks is accurate to 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 five days compared to LMP-based dating. The American College of Obstetricians and Gynecologists considers any pregnancy without an ultrasound before 22 weeks to be “suboptimally dated.”
What “Full Term” Actually Means
Not every delivery at or near 40 weeks carries the same outlook. The National Institutes of Health and major obstetric organizations break late pregnancy into four 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. Babies born at 37 or 38 weeks are sometimes treated as “term,” but they have slightly higher rates of breathing problems and feeding difficulties compared to those born at 39 or 40 weeks. The sweet spot is that full-term window of 39 weeks to 40 weeks and 6 days.
The Three Trimesters
The first trimester runs from the first day of your LMP through 13 weeks and 6 days. This is when all major organs form. The second trimester spans weeks 14 through 27 and is a period of rapid growth. By the third trimester, from week 28 to delivery, the fetus is primarily gaining weight and its organs are maturing in preparation for life outside the womb.
Normal Variation Is Wider Than You’d Think
Even among healthy, uncomplicated pregnancies with no induction, gestation length varies quite a bit. One well-known study that pinpointed the exact day of ovulation found a 37-day spread in delivery dates, even after excluding preterm births and complications. That means two perfectly healthy pregnancies could differ by more than five weeks.
Several factors influence where you fall in that range. Maternal age plays a role: older mothers tend to carry slightly longer. Whether you’ve given birth before matters too, as first pregnancies often run a few days longer than subsequent ones. Race and ethnicity also correlate with differences in gestational length. Black women in the United States deliver an average of about five days earlier than white women, a gap that researchers attribute primarily to lasting socioeconomic disadvantage and discrimination rather than genetics.
Twins and Multiples Arrive Earlier
The 40-week figure applies to singleton pregnancies. If you’re carrying multiples, expect a shorter gestation. CDC data shows the averages clearly: singletons arrive around 38.7 weeks, twins at 35.2 weeks, and triplets at 31.9 weeks. Quadruplets average just 29.8 weeks. This earlier timing is normal for multiples, and prenatal care is adjusted accordingly.
What Happens When Pregnancy Goes Past the Due Date
About 10% of pregnancies extend past 41 weeks. Going a few days beyond your due date is common and usually fine, but true post-term pregnancy (42 weeks and beyond) carries real risks. These include a drop in amniotic fluid levels, which can compress the umbilical cord and reduce oxygen flow to the baby. The placenta also becomes less efficient at delivering nutrients. There’s a higher chance of the baby being unusually large, which increases the likelihood of a cesarean or assisted delivery. Stillbirth risk, while still small in absolute terms, rises measurably after 42 weeks.
For these reasons, labor induction is typically recommended if pregnancy reaches 41 weeks. Your provider will generally begin more frequent monitoring, including checks on amniotic fluid and fetal heart rate, once you pass your due date.
Why Nine Months and Not Longer
For decades, anthropologists explained human gestation length with the “obstetrical dilemma” hypothesis: the baby’s head needs to fit through a pelvis shaped for walking upright, so birth has to happen before the skull gets too large. More recent research challenges that idea. A competing hypothesis argues that the real limit is maternal metabolism. By the end of pregnancy, the energy demands of the fetus approach the maximum a mother’s body can sustain. Birth happens not because the baby’s head runs out of room, but because the mother’s metabolism hits a ceiling. This helps explain why human newborns are so helpless compared to other primates: they’re born at the point of metabolic constraint, not at some optimal stage of brain development.