A pregnancy lasts 40 weeks, or 280 days, counted from the first day of your last menstrual period. That number surprises many people because it includes roughly two weeks before conception actually happens. The baby’s true time in the womb is closer to 38 weeks, but the 40-week standard is what doctors use to set your due date and track development.
Why the Count Starts Before Conception
Pregnancy is dated from the first day of your last period, not from the day you conceived. Since ovulation and fertilization typically happen about two weeks into a menstrual cycle, the first two “weeks of pregnancy” occur before sperm ever meets egg. This convention exists because most people can pinpoint when their period started but not exactly when they ovulated. The standard formula, 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. It assumes a 28-day cycle, so if your cycles are longer or shorter, your actual due date may shift.
When measured from the known date of ovulation rather than the last period, the median pregnancy length is 268 days, or 38 weeks and 2 days. That’s the actual biological duration of gestation for most pregnancies.
How Accurate Your Due Date Really Is
Only about 4 to 5 percent of babies arrive on their predicted due date. About two-thirds are born within a week of it, but even among healthy pregnancies with no complications, the natural variation in length spans roughly 37 days. Your due date is best understood as the center of a window, not a fixed appointment.
A first-trimester ultrasound is the most reliable way to pin down gestational age. By measuring the embryo’s length in the first 13 weeks, doctors can estimate the due date within plus or minus 5 to 7 days. That’s accurate enough that in one study, 40 percent of women who received an early ultrasound had their due date adjusted because it disagreed with their period-based calculation by more than five days. If you had irregular cycles, weren’t sure of your last period, or conceived while on birth control, ultrasound dating is especially important.
Early Term, Full Term, and Beyond
Not all weeks at the end of pregnancy are equal. Doctors now use four specific categories instead of lumping everything from 37 to 42 weeks together as “term”:
- 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 significant development happens in those final weeks. At week 38, a baby typically weighs around 6.5 pounds, though some reach nearly 9 pounds. Fat continues building through week 39 to help regulate body temperature after birth, and the chest cavity is still expanding. By week 40, the average baby measures about 14 inches from crown to rump and weighs roughly 7.5 pounds. Babies born at 37 weeks are far more likely to need extra support than those born at 39 weeks, which is why elective deliveries are discouraged before 39 weeks unless there’s a medical reason.
If your pregnancy reaches 41 weeks, your provider will likely discuss induction. Post-term pregnancies carry increased risks for both mother and baby, so most guidelines recommend induction by 41 weeks.
What Affects How Long Your Pregnancy Lasts
Several factors nudge pregnancy length in one direction or the other. First-time mothers tend to carry slightly longer, averaging 275.9 days compared to 274.5 days for women who have given birth before. That’s only about a day and a half, but it’s a consistent pattern across large datasets. The old saying that first babies are “always late” has a grain of truth, though the difference is modest.
Twin pregnancies are substantially shorter. Most twins arrive around 36 weeks, with a typical range of 32 to 38 weeks depending on whether the twins share a placenta. This isn’t a complication; it’s the expected timeline, and prenatal care for twins is planned accordingly.
Maternal age and weight also play a role, though their influence is more about risk than simple duration. Higher body weight combined with age over 35 increases the likelihood of preterm delivery, particularly very early preterm birth before 28 weeks. These are statistical risks, not certainties, but they’re part of why prenatal visits include weight and blood pressure monitoring throughout pregnancy.
What 40 Weeks Looks Like Trimester by Trimester
The 40 weeks divide into three trimesters, each with a different character. The first trimester covers weeks 1 through 12. During this stretch, the fertilized egg implants, the placenta forms, and all major organ systems begin to develop. Most pregnancy symptoms like nausea and fatigue peak during this period, and the risk of miscarriage is highest.
The second trimester runs from week 13 through week 27. This is when many people feel their best physically. The baby grows from a few inches long to over a foot, begins moving noticeably, and develops the ability to hear. Anatomy scans typically happen around week 20, giving the first detailed look at the baby’s structure.
The third trimester, weeks 28 through 40, is when the baby gains most of its weight and the lungs finish maturing. The fine hair covering the baby’s body disappears by around week 38, the toenails reach the tips of the toes, and the brain is undergoing rapid development. For you, this trimester brings increasing discomfort as the baby takes up more space, along with more frequent prenatal visits to monitor blood pressure, fetal position, and growth.
Counting Your Own Weeks
If you know the date of your last period, you can estimate your current week by counting forward from that date. Most pregnancy apps and online calculators do this automatically. Keep in mind that your provider may adjust the timeline after an early ultrasound, so the “official” gestational age on your chart might differ from what you calculated at home by a few days to a week. Once your due date is established in the first trimester, it typically stays fixed for the rest of your care, even if later ultrasounds suggest slightly different measurements.