Pregnancy lasts about 9 calendar months, or roughly 40 weeks from the first day of your last menstrual period. That 40-week number is what causes confusion, because 40 weeks sounds like it should be 10 months. The reality is simpler than it seems once you understand how months and weeks actually line up.
Why 40 Weeks Isn’t 10 Months
Most people assume a month equals exactly 4 weeks, so they multiply 40 weeks by 4 and land on 10 months. But calendar months are longer than 28 days. The average month is actually about 4.35 weeks, since most months have 30 or 31 days. When you divide 40 weeks by 4.35, you get roughly 9.2 months. That lines up with the familiar “about 9 months” answer.
There’s another layer to this. Pregnancy is counted from the first day of your last period, not from the day you actually conceived. Ovulation and conception typically happen about two weeks into your cycle. So while the clock starts ticking at 40 weeks before your due date, the baby has only been developing for about 38 weeks (266 days). That 38-week figure is closer to 8 months and 3 weeks of actual fetal growth.
How the Three Trimesters Break Down
Pregnancy is divided into three trimesters, each covering roughly three calendar months:
- First trimester: Weeks 4 through 12. This covers approximately months 1 through 3. (Weeks 1 through 3 are before and around conception, so most people don’t even know they’re pregnant yet.)
- Second trimester: Weeks 13 through 27, covering months 4 through 6. This is when nausea typically fades and energy returns for many people.
- Third trimester: Weeks 28 through 41, covering months 7 through 9 (and sometimes a bit beyond).
These ranges aren’t perfectly tidy because months and weeks don’t divide evenly. Different sources will shift the boundaries by a week in either direction. The general shape is always the same: three roughly equal blocks of about 13 weeks each.
Why the Due Date Is an Estimate
The standard due date formula dates back to the 1850s, when German obstetrician Franz Naegele calculated that the average pregnancy lasted about 280 days (40 weeks) from the start of the last menstrual period. His formula assumed a 28-day cycle with ovulation on day 14. It’s still the basis for due date calculations today, even though many people have cycles that are shorter or longer than 28 days.
A study published through the European Society of Human Reproduction and Embryology tracked pregnancies from the confirmed day of ovulation and found that the average length from ovulation to birth was 268 days, or 38 weeks and 2 days. More striking, the natural variation among healthy pregnancies spanned as much as 37 days, nearly five full weeks. Two people who conceive on the same day could deliver more than a month apart, and both pregnancies would be completely normal.
Because of this wide range, very few babies arrive on their exact due date. Induction is also common at or near 40 weeks in many countries, which makes it difficult to pin down what percentage of pregnancies would naturally hit that target. The due date is best understood as the middle of a window, not a deadline.
What Counts as “Full Term”
Not all weeks near the end of pregnancy are treated the same. The American College of Obstetricians and Gynecologists classifies 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 even a week or two early can have different outcomes than those born at 39 or 40 weeks. A baby born at 37 weeks is not premature by the old definition, but “early term” signals that those last couple of weeks still contribute to lung, brain, and liver development. This is why most providers prefer not to schedule elective deliveries before 39 weeks unless there’s a medical reason.
A Quick Month-by-Month Reference
If you want a rough conversion from weeks to months, here’s how it maps:
- Month 1: Weeks 1–4
- Month 2: Weeks 5–8
- Month 3: Weeks 9–13
- Month 4: Weeks 14–17
- Month 5: Weeks 18–22
- Month 6: Weeks 23–27
- Month 7: Weeks 28–31
- Month 8: Weeks 32–35
- Month 9: Weeks 36–40
These ranges are approximate. You’ll notice they don’t divide into neat 4-week blocks, and that’s the point: calendar months are slightly longer than 4 weeks, so the weeks accumulate faster than people expect. If someone tells you they’re 32 weeks pregnant, that’s about 7 and a half months, not 8. The week count is always more precise than the month count, which is why doctors and midwives track pregnancy in weeks rather than months.