A typical pregnancy lasts 280 days, or 40 weeks, counted from the first day of your last menstrual period. That’s the standard number used to calculate your due date, but it’s not the whole story. The actual number of days your pregnancy lasts depends on when you ovulated, whether this is your first baby, and plain biological variation that differs from person to person.
Where the 280-Day Number Comes From
The 280-day figure is based on a calculation method called Naegele’s Rule, which assumes a 28-day menstrual cycle with ovulation happening on day 14. To estimate your due date using this method, you take the first day of your last period, count back three calendar months, then add one year and seven days. It’s the standard formula used by most healthcare providers.
But here’s the catch: 280 days includes roughly two weeks before you actually conceived. Because ovulation typically happens around day 14 of your cycle, the real time from fertilization to birth is closer to 266 days, or 38 weeks. Doctors still count from the last menstrual period because most people can recall when their period started, while the exact day of conception is harder to pin down.
If your cycle is longer or shorter than 28 days, or if you ovulated earlier or later than day 14, the 280-day estimate shifts accordingly. That’s one reason early ultrasounds are used to refine your due date, especially when your cycle length is irregular.
How Much Pregnancy Length Actually Varies
Even among healthy pregnancies with no complications, the length varies more than most people expect. A study published in Human Reproduction tracked women whose exact ovulation dates were known and found that pregnancy length spanned a range of 37 days, even after excluding preterm births. The median time from ovulation to birth was 268 days (38 weeks and 2 days), but some women delivered nearly three weeks earlier or later than others with no medical explanation.
First-time mothers tend to carry slightly longer. An analysis of delivery records found that the average pregnancy for a first birth was 275.9 days, compared to 274.5 days for women who had delivered before. That’s only about a day and a half difference, but it lines up with what many parents experience: first babies often arrive a little late.
Only about 4% of babies arrive on their actual due date. Your due date is best understood as the middle of a window, not a deadline.
The Three Trimesters in Days and Weeks
Pregnancy is divided into three trimesters, each with a distinct phase of development:
- First trimester: Day 1 through 13 weeks and 6 days (roughly the first 98 days). This is when fertilization occurs and all major organs begin forming.
- Second trimester: 14 weeks through 27 weeks and 6 days (roughly days 99 through 196). The baby grows rapidly in size and you’ll typically start feeling movement.
- Third trimester: 28 weeks through 40 weeks and 6 days (roughly days 197 through 287). The baby gains weight and the organs mature in preparation for life outside the womb.
What “Full Term” Really Means
The phrase “full term” has a more specific meaning than most people realize. The American College of Obstetricians and Gynecologists breaks it into four categories:
- Early term: 37 weeks through 38 weeks and 6 days (days 259 through 272)
- Full term: 39 weeks through 40 weeks and 6 days (days 273 through 286)
- Late term: 41 weeks through 41 weeks and 6 days (days 287 through 293)
- Post-term: 42 weeks and beyond (day 294+)
These distinctions matter because babies born even a week or two early can face different outcomes than those born at 39 weeks or later. The “full term” window of 39 to 40 weeks is when the baby’s brain, lungs, and liver have had the most time to mature.
What Happens if Pregnancy Goes Past 40 Weeks
Going past your due date is common, but going significantly past it carries risks. Once a pregnancy reaches 41 weeks, your provider will likely begin monitoring more closely with tests that check the baby’s heart rate, movement, and amniotic fluid levels.
After 42 weeks, the risks increase. The baby may grow larger than average, making delivery more difficult and raising the chance of a C-section or assisted delivery. Amniotic fluid can drop to low levels, which may compress the umbilical cord during contractions and affect the baby’s heart rate. For the mother, a post-term pregnancy raises the likelihood of severe vaginal tears, infection, and heavier postpartum bleeding. Based on monitoring results, your provider may recommend inducing labor before these risks escalate.
Why Your Due Date Is an Estimate
The 280-day calculation assumes you know the exact start of your last period, that your cycle is 28 days, and that you ovulated on day 14. In reality, few people fit that template perfectly. Cycle lengths vary, ovulation timing shifts from month to month, and many people don’t track their periods precisely enough to give an exact start date.
When researchers measured pregnancy length from the last menstrual period versus from confirmed ovulation, the variation was noticeably larger using the period-based method (a coefficient of variation of 4.9% versus 3.7%). That extra variability comes entirely from the guesswork around when ovulation actually happened. First-trimester ultrasounds help narrow things down by measuring the baby’s size, and most providers will adjust your due date if the ultrasound estimate differs significantly from the one calculated by your period.
The bottom line: 280 days is the number to know, but your baby’s actual birthday will fall somewhere in a window around it. Anything between 37 and 42 weeks is within the range of normal human pregnancy.