A full-term pregnancy lasts about 40 weeks, which works out to roughly nine calendar months. But converting weeks to months isn’t as straightforward as it sounds, because calendar months aren’t all the same length. Most have 30 or 31 days rather than a neat 28, so the weeks don’t divide evenly. Here’s how the math actually works and why your doctor counts in weeks instead of months.
Why Weeks and Months Don’t Line Up Neatly
If every month were exactly four weeks, 40 weeks would equal 10 months. But pregnancy is dated from the first day of your last menstrual period, not from conception, and conception typically happens about two weeks later. So the actual time a fetus develops is closer to 38 weeks (about 267 days on average, with a standard deviation of roughly 10 days). That gap between “gestational age” and “fetal age” is one reason the month count feels confusing.
The other reason is that calendar months average about 4.3 weeks, not 4. When you account for that, 40 gestational weeks comes out to just over nine months. Doctors avoid this messiness entirely by tracking pregnancy in weeks and days, which is far more precise for monitoring development.
Weeks to Months: A Practical Breakdown
There’s no single official conversion, but the ranges below are widely used and align with how trimesters are defined clinically.
- Month 1: Weeks 1 through 4
- Month 2: Weeks 5 through 8
- Month 3: Weeks 9 through 13
- Month 4: Weeks 14 through 17
- Month 5: Weeks 18 through 21
- Month 6: Weeks 22 through 26
- Month 7: Weeks 27 through 30
- Month 8: Weeks 31 through 35
- Month 9: Weeks 36 through 40
You’ll notice month 3 spans five weeks. That’s because the first trimester officially runs through 13 weeks and 6 days, and squeezing 14 weeks into three equal months doesn’t work perfectly. Different sources shift the boundaries by a week in either direction, which is why you might get a slightly different answer depending on where you look. The important thing is that your provider tracks your exact week, not the month.
The Three Trimesters
Trimesters are defined more precisely than months. The American College of Obstetricians and Gynecologists (ACOG) breaks them down this way:
- First trimester: First day of your last period through 13 weeks, 6 days. This is when major organs begin forming.
- Second trimester: 14 weeks, 0 days through 27 weeks, 6 days. A period of rapid growth, and when most people start to feel movement.
- Third trimester: 28 weeks, 0 days through 40 weeks, 6 days. The fetus gains weight and organs mature in preparation for birth.
How Your Due Date Is Calculated
The standard method, known as Naegele’s Rule, uses a simple formula: take the first day of your last menstrual period, count back three calendar months, then add one year and seven days. So if your last period started on March 10, you’d count back to December 10 and add a year and a week, giving you a due date of December 17. This assumes a 28-day cycle, so if your cycles are longer or shorter, the estimate shifts.
An early ultrasound refines that estimate. In the first trimester, ultrasound dating is accurate to within 5 to 7 days. By the second trimester, accuracy drops, and third-trimester ultrasounds can be off by 21 to 30 days. That’s why the dating scan done around 8 to 12 weeks carries the most weight.
What “Full Term” Actually Means
Not all deliveries at or after 37 weeks are considered the same. ACOG uses four categories to distinguish outcomes more precisely:
- Early term: 37 weeks, 0 days through 38 weeks, 6 days
- Full term: 39 weeks, 0 days through 40 weeks, 6 days
- Late term: 41 weeks, 0 days through 41 weeks, 6 days
- Post-term: 42 weeks, 0 days and beyond
These distinctions matter because babies born at 39 weeks tend to have better outcomes than those born at 37, even though both fall in the broad “term” window. If your pregnancy reaches 41 weeks without labor starting on its own, your provider will typically discuss induction. Most pregnancies don’t go past 42 weeks.
Why Pregnancy Length Varies
A study that tracked pregnancies from the precise day of ovulation found that birth timing ranged from about 29 weeks and 5 days all the way to 40 weeks and 4 days. Among healthy, uncomplicated pregnancies, the average was 268 days from ovulation (38 weeks, 2 days), with a spread of about 9 days in either direction. That means two perfectly normal pregnancies can differ by nearly three weeks.
Your due date is a midpoint of a range, not a deadline. Only about 5% of babies arrive on their exact due date. So when someone asks how many months pregnant you are, the honest answer is almost always an approximation, and that’s completely fine. What matters is the week-by-week tracking your provider uses to monitor growth, schedule screenings, and plan for delivery.