Pregnancy weeks to months converter
Convert how many weeks pregnant you are into months — or the other way round. Because 40 weeks equals 9 months, the boundaries aren’t a simple divide-by-four, so this converter uses the standard obstetric mapping.
How to use it
Pregnancy weeks to months chart
Because a pregnancy month is a little longer than four weeks, here is the standard mapping used above — a quick reference for any week:
| Pregnancy month | Gestational weeks | Trimester |
|---|---|---|
| Month 1 | 0–4½ weeks | First trimester |
| Month 2 | 4½–9 weeks | First trimester |
| Month 3 | 9–13½ weeks | First trimester |
| Month 4 | 13½–18 weeks | Second trimester |
| Month 5 | 18–22½ weeks | Second trimester |
| Month 6 | 22½–27 weeks | Second trimester |
| Month 7 | 27–31½ weeks | Third trimester |
| Month 8 | 31½–36 weeks | Third trimester |
| Month 9 | 36–40 weeks | Third trimester |
Why 40 weeks is 9 months (not 10)
Four weeks would make a 28-day month, but calendar months average about 30.4 days. Over a whole pregnancy that difference adds up: 40 weeks is roughly 9.2 calendar months, which is why pregnancy is described as nine months even though 40 ÷ 4 would suggest ten. The weeks are the precise measure; months are the friendly approximation. For where those weeks come from, see how many weeks and days you’re pregnant for.
From weeks to a due date
This converter handles weeks and months; to turn a week count into calendar dates — your due date, the date you’ll reach a given week, or how far along you are today — use the gestational age calculator or the full OB wheel.
Frequently asked questions
How many months is 16 weeks pregnant?
16 weeks is 4 months pregnant — the start of month 4 and the second trimester. Pregnancy months run a little longer than four calendar weeks, so the mapping isn’t a simple divide-by-four.
How many months is 28 weeks pregnant?
28 weeks is 7 months pregnant and the start of the third trimester. Each pregnancy month spans roughly 4 to 4.5 weeks.
How many months is 40 weeks?
40 weeks is 9 months — the full length of a pregnancy counted from the last period. It’s a little over nine calendar months because weeks don’t divide evenly into months.
Why don’t pregnancy weeks divide evenly into months?
Because a calendar month is longer than four weeks. Forty weeks is about 9.2 calendar months, so the standard convention maps 40 weeks onto 9 pregnancy months with slightly uneven boundaries.
How do I convert pregnancy weeks to months?
Use the converter above, or the reference table: months begin at roughly weeks 0, 4.5, 9, 13.5, 18, 22.5, 27, 31.5 and 36. Enter your weeks and it does the rest.
Last meaningfully updated: 2026-08-25.