Pregnancy Weeks to Months Calculator
Enter how many weeks pregnant you are to convert that into months and trimester — a conversion that trips people up often, since a pregnancy month isn't a clean 4 weeks.
How the Pregnancy Weeks to Months formula works
A full-term pregnancy is about 40 weeks across roughly 9 months, which averages to about 4.35 weeks per month:
Months ≈ Weeks / 4.345
Step-by-step calculation
- Divide the number of weeks pregnant by approximately 4.345 (40 weeks ÷ 9.2 months) to estimate months.
- Compare the week count against trimester boundaries: weeks 1–13 are the first trimester, 14–27 the second, and 28–40 the third.
Worked example
20 weeks pregnant: Months ≈ 20 / 4.345 ≈ 4.6 months — roughly the end of the 4th month or start of the 5th, and squarely in the second trimester.
Frequently asked questions
Why isn't a pregnancy month exactly 4 weeks?
Because 40 weeks doesn't divide evenly by 4 into a whole number of months — using a flat 4-weeks-per-month assumption would only add up to 36 weeks over 9 months, several weeks short of a full-term pregnancy, so months work out to slightly more than 4 weeks each on average.
Is 40 weeks the same as 9 months exactly?
It's commonly rounded to '9 months' in casual conversation, but 40 weeks is closer to 9.2 months — which is exactly why week counts and month counts don't line up as cleanly as they seem like they should.