Pregnancy lasts about 9 calendar months, or 40 weeks from the first day of your last menstrual period. That 40-week number is the standard used by doctors and midwives, but converting it into months is where things get confusing, because 40 weeks doesn’t divide neatly into calendar months.
Why 40 Weeks Doesn’t Equal Exactly 9 Months
If you do the math with four-week blocks, 40 weeks comes out to 10 months, not 9. That’s because a “pregnancy month” is exactly 4 weeks long, while most calendar months are 4 to 5 weeks. February has 28 days (exactly 4 weeks), but months like January and March have 31 days, adding extra days that accumulate over nine months.
So when someone says pregnancy is 9 months, they’re using calendar months. When a pregnancy app says you’re “8 months pregnant” at 32 weeks, it’s using 4-week blocks. Both are correct in their own system, which is exactly why the medical world skips months entirely and counts in weeks.
How Weeks Map to Each Month
Here’s a general month-by-month breakdown using 4-week increments:
- Month 1: Weeks 1 through 4
- Month 2: Weeks 5 through 8
- Month 3: Weeks 9 through 12
- Month 4: Weeks 13 through 17
- Month 5: Weeks 18 through 22
- Month 6: Weeks 23 through 27
- Month 7: Weeks 28 through 31
- Month 8: Weeks 32 through 36
- Month 9: Weeks 37 through 40
You’ll notice not every “month” is exactly four weeks. Some sources stretch month 4 and month 5 to five weeks each to make everything fit within 40 weeks. There’s no single official standard for this, which is another reason your doctor will always refer to your week number instead.
The Three Trimesters
Trimesters are a more consistent way to track pregnancy than months. The first trimester covers weeks 4 through 12, roughly the first three months. The second trimester runs from week 13 through week 27. The third trimester starts at week 28 and continues until delivery, typically around week 40 or 41.
Each trimester is loosely three months long, but because of the week-to-month mismatch, the boundaries don’t line up perfectly with calendar months either. If someone asks how far along you are, giving your week number and trimester is the clearest answer.
Why Pregnancy Starts Before Conception
One thing that surprises many people: the 40-week clock starts on the first day of your last menstrual period, not the day you actually conceived. Ovulation and fertilization typically happen around day 14 of a 28-day cycle, so by the time a sperm meets an egg, you’re already considered “2 weeks pregnant.” By the time you miss a period and get a positive test, you’re roughly 4 weeks along.
This convention exists because most people can identify the start of their last period more reliably than the exact day they ovulated. Doctors calculate a due date by counting back 3 calendar months from that date and then adding 1 year and 7 days. On a standard 28-day cycle, that formula lands on 280 days, or 40 weeks. If your cycles are irregular or you’re unsure of the date, an early ultrasound can pin down the timing more accurately.
When Pregnancy Is Considered “Full Term”
Not all weeks near the finish line are equal. The American College of Obstetricians and Gynecologists breaks the final stretch into distinct 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
A baby born at 37 weeks is not premature, but those last two weeks before the 39-week mark matter for lung and brain development. The ideal window for delivery is between 39 weeks, 0 days and 40 weeks, 6 days.
How Long Most Pregnancies Actually Last
The 40-week target is an estimate, not a guarantee. A large analysis of delivery records found that the average pregnancy lasts about 275.9 days (roughly 39 weeks and 3 days) for first-time mothers, and 274.5 days for those who have given birth before. First babies tend to take a little longer: 6.2% of first-time deliveries go past 41 weeks, compared to only 4% for mothers who have delivered previously.
So while 40 weeks is the benchmark, most babies arrive a few days before that mark. Only about 5% of babies are born on their exact due date. Anything between 37 and 42 weeks falls within the normal range, which in calendar terms spans from about 8.5 months to just over 9.5 months.