A full-term pregnancy lasts 40 weeks, counted from the first day of your last menstrual period. Most pregnancies end somewhere between 37 and 42 weeks, and the sweet spot for delivery is 39 weeks through 40 weeks and 6 days.
Why Pregnancy Is Counted as 40 Weeks
The 40-week number surprises some people because it doesn’t start at conception. Pregnancy is dated from the first day of your last menstrual period, which is roughly two weeks before you actually conceive. That means a “40-week” pregnancy includes about 38 weeks of actual fetal development. This system exists because most people can pinpoint their last period more reliably than the exact day of conception.
Your due date is calculated using a simple formula: take the first day of your last period, count back three calendar months, then add one year and seven days. This method assumes a 28-day menstrual cycle. If your cycles are longer or shorter, or if the exact date is uncertain, an early ultrasound can adjust the estimate.
What “Full Term” Actually Means
Not all deliveries between 37 and 42 weeks are equal. The National Institutes of Health and major obstetric organizations break that window into 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
These categories matter because babies born at 39 or 40 weeks generally have better outcomes than those born at 37 or 38 weeks. Those last two weeks allow important development in the brain, lungs, and liver. That’s why major guidelines discourage elective delivery before 39 weeks unless there’s a medical reason to deliver earlier.
When Most People Actually Deliver
Your due date is an estimate, not an appointment. Only a small percentage of babies arrive on that exact day. For first-time mothers with low-risk pregnancies, the median point for spontaneous labor is about 40.1 weeks, and roughly 80% go into labor by 41 weeks. The most common window is between 40 weeks 0 days and 40 weeks 6 days.
If you’ve had a baby before, labor tends to start a bit earlier, though the difference is usually just a few days. Every pregnancy has its own timeline, and a range of a few weeks in either direction is normal.
Twin and Triplet Pregnancies Are Shorter
If you’re carrying multiples, expect a shorter pregnancy. Most twin pregnancies deliver at around 36 weeks, with a typical range of 32 to 38 weeks depending on the type of twin pregnancy (whether the babies share a placenta matters). This isn’t a complication by itself. Twins simply tend to be ready earlier, and providers plan delivery timing accordingly.
What Happens After 41 Weeks
Going past your due date is common and usually not dangerous. But the risks do increase gradually once a pregnancy reaches late-term or post-term status. After 42 weeks, potential complications include decreased amniotic fluid (which can compress the umbilical cord and reduce oxygen flow), the baby passing its first stool before birth (which can cause breathing problems if inhaled), and a higher chance of stillbirth. The baby may also grow larger than expected, increasing the likelihood of a cesarean delivery or assisted vaginal delivery. For the mother, post-term pregnancy raises the chance of infection and heavy bleeding after birth.
These problems occur in only a small number of post-term pregnancies, but they’re the reason most providers will discuss induction if you reach 41 or 42 weeks. Your provider will typically monitor you more closely with additional ultrasounds or heart-rate checks as you approach and pass your due date.
How Weeks Translate to Months and Trimesters
The weeks-to-months math in pregnancy never lines up neatly because calendar months aren’t exactly four weeks long. A rough breakdown:
- First trimester: Weeks 1 through 12 (months 1 through 3)
- Second trimester: Weeks 13 through 27 (months 4 through 6)
- Third trimester: Weeks 28 through 40 (months 7 through 9)
This is why you’ll sometimes hear pregnancy described as “nine months” and other times as “ten months.” Forty weeks is technically closer to nine calendar months and one week. Providers almost always use weeks rather than months because weeks are more precise, and a single week can make a real difference in how a baby’s development is assessed.