A typical pregnancy lasts 40 weeks, or 280 days, counted from the first day of your last menstrual period. That’s about 9 calendar months, though you’ll hear doctors and midwives talk almost exclusively in weeks because it’s more precise. Only about 5% of babies arrive on their exact due date, so the 40-week mark is a target, not a guarantee.
Why the Count Starts Before Conception
One of the most confusing things about pregnancy math is that the clock starts roughly two weeks before you actually conceive. Doctors count from the first day of your last period because most people can remember that date, while the exact day of ovulation and fertilization is harder to pin down. This means that during “week one” and “week two” of pregnancy, you aren’t pregnant yet. Actual fetal development spans closer to 38 weeks.
This counting method, called Naegele’s Rule, assumes a 28-day menstrual cycle with ovulation on day 14. If your cycles are longer or shorter, or if you ovulate earlier or later than average, the standard 40-week estimate may be slightly off for you. That’s one reason early ultrasounds are commonly used to refine the due date.
How to Estimate Your Due Date
You can calculate a rough due date yourself in three steps: take the first day of your last period, count back three calendar months, then add one year and seven days. If your last period started on March 10, for example, you’d count back to December 10, then add a year and seven days to land on December 17 of the following year.
This formula works best for people with regular 28-day cycles. Your provider will likely adjust the estimate with an early ultrasound, especially if your cycles are irregular or you aren’t sure when your last period began.
The Three Trimesters
Pregnancy is divided into three trimesters, each with a distinct phase of development:
- First trimester (weeks 1 through 13): Fertilization occurs, and all major organs begin forming. This is when most people experience nausea, fatigue, and breast tenderness.
- Second trimester (weeks 14 through 27): A period of rapid growth. The fetus starts moving noticeably, and many people find that early symptoms like nausea ease up.
- Third trimester (weeks 28 through 40): The fetus gains weight and its organs mature in preparation for life outside the womb. You’ll feel heavier, sleep may get harder, and Braxton Hicks contractions can begin.
What “Full Term” Actually Means
Not all weeks near the end of pregnancy are equal. The American College of Obstetricians and Gynecologists breaks down the final stretch into four categories:
- Early term: 37 weeks through 38 weeks, 6 days
- Full term: 39 weeks through 40 weeks, 6 days
- Late term: 41 weeks through 41 weeks, 6 days
- Post-term: 42 weeks and beyond
These distinctions matter because babies born at 39 weeks tend to have better outcomes than those born at 37 weeks. The brain, lungs, and liver are still maturing during those final two weeks. That’s why elective deliveries are generally not scheduled before 39 weeks unless there’s a medical reason.
When Pregnancy Goes Past 40 Weeks
Going past your due date is common. Most pregnancies that extend beyond 40 weeks still result in healthy deliveries, but the risks increase as you approach 42 weeks. After that point, there may be less amniotic fluid around the baby, and the baby is more likely to be larger than average, which can complicate delivery.
Providers typically discuss induction if labor hasn’t started on its own by one to two weeks past the due date. At 42 weeks, the pregnancy is officially classified as post-term, and most providers will recommend induction at or before that point.
Why Pregnancy Length Varies
While 40 weeks is the standard benchmark, individual pregnancies can vary by several weeks in either direction. Research from Brigham and Women’s Hospital found that the most important factors influencing pregnancy length are whether you’ve given birth before, your age, and your race. People who have previously given birth, those younger than 19 or older than 34, and Black women tend to have slightly shorter pregnancies on average compared to first-time mothers between 19 and 34.
These variations are normal. A healthy pregnancy can last anywhere from 37 to 42 weeks. Your provider tracks your specific timeline through regular appointments, adjusting expectations based on your body and your baby’s development rather than relying on a single fixed number.