A typical pregnancy lasts 280 days, or 40 weeks, counted from the first day of your last menstrual period. That’s the number your doctor uses to calculate a due date and track your progress. The actual time a baby spends developing after conception is shorter: about 266 days, or 38 weeks. The difference exists because pregnancy counting begins roughly two weeks before fertilization occurs.
Why Pregnancy Is Counted From Your Last Period
Most people don’t know the exact day they conceived, but most can identify when their last period started. That date becomes “day 1” of pregnancy, even though fertilization typically happens around day 14 of a 28-day menstrual cycle. This system means you’re already considered “two weeks pregnant” at the point of conception.
The standard formula for calculating a due date, known as Naegele’s Rule, works like this: take the first day of your last period, count back three calendar months, then add one year and seven days. Johns Hopkins Medicine notes this assumes a 28-day menstrual cycle, so if your cycle is longer or shorter, your due date may shift accordingly. An early ultrasound can refine the estimate by measuring the embryo directly.
For people who conceive through IVF, the math is more precise because the date of embryo transfer is known. Fertility clinics use that transfer date as the starting point instead of relying on menstrual cycle estimates.
How Much Pregnancy Length Varies
The 280-day figure is an average, not a guarantee. A large Norwegian study of over 309,000 pregnancies found the median length was 283 days from the last menstrual period, with labor starting on its own. Measured from conception, the average is 266 days with a standard deviation of 16 days. That means roughly two-thirds of pregnancies last between 250 and 282 days from fertilization, or about 36 to 40 weeks from the last period.
Many factors influence where you fall in that range. First pregnancies tend to run a bit longer. Your age, genetics, and the length of your natural menstrual cycle all play a role. A healthy pregnancy that delivers at 38 weeks is just as normal as one that reaches 41 weeks.
The Three Trimesters in Days
The American College of Obstetricians and Gynecologists divides pregnancy into three trimesters, each with a distinct phase of development:
- First trimester: Day 1 through 13 weeks and 6 days (roughly days 1 to 97). This is when fertilization occurs and the major organs form.
- Second trimester: 14 weeks through 27 weeks and 6 days (roughly days 98 to 195). The baby grows rapidly and you typically start feeling movement.
- Third trimester: 28 weeks through 40 weeks and 6 days (roughly days 196 to 286). The baby gains weight and the organs mature in preparation for birth.
What “Full Term” Actually Means
Not all deliveries between 37 and 42 weeks carry the same risks. The National Institutes of Health helped redefine “term pregnancy” into more specific categories:
- Early term: 37 weeks through 38 weeks and 6 days (days 259 to 272)
- Full term: 39 weeks through 40 weeks and 6 days (days 273 to 286)
- Late term: 41 weeks through 41 weeks and 6 days (days 287 to 293)
- Post-term: 42 weeks and beyond (day 294 onward)
These distinctions matter because babies born in the “full term” window of 39 to 40 weeks generally have the best outcomes. Babies born at 37 weeks, while not premature, have slightly higher rates of breathing difficulties and feeding problems compared to those born just two weeks later.
What Happens When Pregnancy Goes Past 280 Days
About 1 in 10 pregnancies extends past the due date. Going a few days beyond 280 days is common and usually not a concern on its own. But as a pregnancy stretches past 41 weeks (287 days), the placenta gradually becomes less efficient at delivering oxygen and nutrients. The risk of complications rises slowly from that point.
When you’re more than a week past your due date, your provider will typically begin closer monitoring. According to the Mayo Clinic, this often includes a test measuring the baby’s heart rate and an ultrasound to check heart rate, breathing, muscle tone, movement, and amniotic fluid levels. Based on those results, your provider may recommend inducing labor. Most guidelines suggest induction somewhere between 41 and 42 weeks to reduce the risks associated with post-term pregnancy.
Quick Reference: Pregnancy by the Numbers
- From last menstrual period: 280 days (40 weeks, 9 calendar months)
- From conception: 266 days (38 weeks)
- Normal range: roughly 250 to 285 days from conception
- Full term sweet spot: 39 to 40 weeks (273 to 286 days from last period)
- Post-term threshold: 42 weeks (294 days from last period)