Pregnancy lasts about 40 weeks, which works out to roughly 10 lunar months (each 4 weeks long) rather than the “9 months” most people expect. The confusion is normal: calendar months average 4.3 weeks, so the weeks don’t divide neatly into nine equal chunks. Once you understand why the math feels off, converting weeks to months becomes straightforward.
Why Pregnancy Is 40 Weeks, Not 9 Months
Pregnancy is counted from the first day of your last menstrual period (LMP), not from the day you actually conceived. Because ovulation typically happens about two weeks into a cycle, the first two weeks of “pregnancy” occur before conception even takes place. This dating convention exists because most people can pinpoint the start of a period far more reliably than the day of conception.
Forty weeks from LMP equals 280 days. Divide that by the average calendar month length of 30.4 days and you get roughly 9 months and 1 week. Divide by exactly 4 weeks per month and you get 10 months. Neither answer is wrong; they just use different definitions of “month.” Doctors avoid the ambiguity entirely by tracking pregnancy in weeks and days.
Weeks Mapped to Months
No single conversion chart is universally official, because calendar months vary in length. The mapping below is widely used by maternal health providers and gives you a practical way to translate your week count into months.
- 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 some months span four weeks and others span five. That’s because the extra days in each calendar month accumulate over nine months, adding about four extra weeks compared to a strict 4-weeks-per-month grid. The months that get five weeks vary depending on the source, so don’t worry if your pregnancy app groups them slightly differently.
How Trimesters Fit In
Trimesters are more standardized than months. The American College of Obstetricians and Gynecologists defines them this way:
- First trimester: First day of LMP through 13 weeks and 6 days
- Second trimester: 14 weeks and 0 days through 27 weeks and 6 days
- Third trimester: 28 weeks and 0 days through 40 weeks and 6 days
In month terms, the first trimester covers roughly months 1 through 3, the second trimester months 4 through 6, and the third trimester months 7 through 9. If your provider mentions a trimester boundary, the week count is the precise marker they’re using.
How to Calculate Your Due Date
The quickest pencil-and-paper method is a formula known as Naegele’s Rule, outlined by Johns Hopkins Medicine in three steps:
- Find the first day of your last menstrual period.
- Count back 3 calendar months from that date.
- Add 1 year and 7 days.
For example, if your last period started June 1, 2025, count back three months to March 1, then add one year and seven days: your estimated due date is March 8, 2026. This formula assumes a 28-day cycle. If your cycles are consistently longer or shorter, shift the due date by the difference. A 35-day cycle, for instance, would push the estimate about a week later.
When Ultrasound Changes the Count
A first-trimester ultrasound (up to 13 weeks and 6 days) is the most accurate way to establish gestational age. If the ultrasound date and the LMP-based date don’t match, your provider will typically use whichever measurement is considered more reliable, and in early pregnancy that’s almost always the ultrasound. This can shift your week count forward or backward by a few days, which in turn changes the month you think you’re in.
The reason early ultrasound is so precise is that embryos grow at a remarkably consistent rate during the first trimester. By the second and third trimesters, individual growth variation widens, so later scans are less useful for resetting a due date.
How IVF Pregnancies Are Dated
If you conceived through in vitro fertilization, the calculation skips the guesswork about ovulation entirely. The date of the embryo transfer and the age of the embryo at transfer are used to assign a gestational age directly. Because there’s no two-week gap between LMP and conception to estimate, an IVF due date calculation is typically about two weeks shorter than the standard LMP method would be for the same conception date. The week-to-month conversion chart above still applies once your provider gives you a gestational age in weeks.
A Simple Way to Keep Track
Rather than converting weeks to months every time someone asks how far along you are, it helps to anchor a few milestones. At 13 weeks you’re finishing your first trimester (about 3 months). At 20 weeks you’re roughly halfway (about 5 months). At 28 weeks you’ve entered the third trimester (about 7 months). At 36 weeks you’re in your ninth month, with about four weeks to go.
When precision matters, stick with weeks. When family and friends ask “how many months?”, use the chart above and don’t stress about minor discrepancies between sources. The weeks-and-days count your provider uses at each visit is the number that actually guides your care.