A normal pregnancy lasts about 40 weeks, or 280 days, counted from the first day of your last menstrual period. That number can be misleading, though, because only about 5% of babies actually arrive on their due date. Most healthy pregnancies end somewhere between 37 and 42 weeks, and the specific week your baby arrives affects how the pregnancy is classified and managed.
Why Pregnancy Is Counted as 40 Weeks
The 40-week count starts from the first day of your last menstrual period, not from the day you actually conceived. Since ovulation typically happens about two weeks into your cycle, you’re already considered “four weeks pregnant” by the time a pregnancy test turns positive. The actual time a baby develops from conception to birth is closer to 38 weeks, but the 40-week convention is what doctors use because the start of your last period is a reliable, trackable date.
This is also why early pregnancy math feels strange. You might find out you’re pregnant at what feels like “two weeks late” and be told you’re already four weeks along.
How Due Dates Are Estimated
Your due date is calculated by adding 280 days to the first day of your last menstrual period. This method has been the standard for over a century. If your cycles are irregular or you aren’t sure of the date, an early ultrasound gives a more precise estimate. Ultrasounds in the second trimester (roughly weeks 14 through 22) are accurate to within 7 to 10 days. Earlier ultrasounds are even more precise because embryos grow at very consistent rates in the first trimester.
For pregnancies conceived through IVF or other assisted reproductive technology, the due date is calculated from the known age of the embryo and the date of transfer, which removes much of the guesswork.
What “Full Term” Actually Means
Not every week within the 37-to-42 range is equal. The National Institutes of Health and the American College of Obstetricians and Gynecologists break it down into four 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. Babies born at 39 or 40 weeks generally have better outcomes than those born at 37 weeks, even though both fall within the “normal” window. The last two weeks before 39 weeks are important for brain development, lung maturity, and building fat stores that help with temperature regulation after birth. This is why elective deliveries are generally not scheduled before 39 weeks unless there’s a medical reason.
Why Pregnancy Length Varies
A range of three to five weeks between “early term” and “post-term” is genuinely normal, and several factors influence where your pregnancy falls in that range.
First-time mothers tend to carry longer than women who have given birth before. Research also shows that first-time mothers face a modestly higher risk of preterm delivery (before 37 weeks), which sounds contradictory but reflects the wider spread of outcomes in a first pregnancy. Your body simply hasn’t done this before, and the timing is less predictable.
Maternal age plays a small but measurable role. Each additional year of age slightly increases the chance of delivering early, with the risk rising by about 4% per year. Genetics matter too. If your mother or sisters carried to 41 weeks, you’re more likely to as well.
Scientists have debated for decades why human pregnancy lasts as long as it does but not longer. One prominent theory suggests that the limit isn’t about the baby’s head fitting through the pelvis, as was long assumed, but about the mother’s metabolism. At a certain point, the energy demands of sustaining the pregnancy approach the upper limit of what the body can maintain. More recent analyses have questioned whether metabolism alone explains it, and the answer is likely a combination of metabolic, hormonal, and mechanical factors.
What Happens After 40 Weeks
Going past your due date is common and usually not a cause for alarm. But the longer a pregnancy continues beyond 41 weeks, the more closely it needs to be monitored. The placenta gradually becomes less efficient at delivering oxygen and nutrients, and the volume of amniotic fluid can decrease. These changes are subtle at first, which is why providers typically increase the frequency of check-ups after 40 weeks.
If your pregnancy reaches 41 weeks, your provider will likely discuss induction. The American College of Obstetricians and Gynecologists recommends considering labor induction at that point to reduce the small but real risks associated with post-term pregnancy, including stillbirth, which rises after 42 weeks. That said, the decision involves a conversation about your specific situation, your preferences, and how you and the baby are doing.
Putting the Numbers in Perspective
If you’re watching the calendar and wondering whether your baby will come “on time,” the honest answer is that the due date is an estimate, not a deadline. The 40-week mark is the center of a bell curve. About half of babies arrive before it and half after. The window of 39 to 41 weeks is where the vast majority of healthy, uncomplicated deliveries happen. Planning for a range rather than a single date tends to make the final weeks of pregnancy less stressful.