Why is pregnancy 40 weeks, not exactly 9 months?
Doctors count pregnancy in weeks from the first day of your last period, not from conception (usually about two weeks later). So a full-term pregnancy is 280 days = 40 weeks, a little more than 9 months. A month isn’t exactly 4 weeks (it’s about 4.3), so some months in the chart have 4 weeks and some 5.
Trimesters
- First trimester: up to the end of week 13 (months 1–3).
- Second trimester: week 14 to the end of week 27 (months 4–6).
- Third trimester: week 28 until birth (months 7–9).
By ACOG’s definitions, birth at 37–38 weeks is early term, 39–40 weeks full term, 41 weeks late term and 42 weeks or more post-term. Few babies arrive exactly on their due date.
Frequently Asked Questions
I’m 24 weeks pregnant. What month is that?
The 6th month: in the common chart, month 6 runs from week 23 to week 27.
Why do other apps show a different month?
There is no single agreed split. Some divide 40 weeks into 9 equal months, others count 4 weeks per month. The number of weeks is what matters, because that is what doctors use.
How do I find my exact due date?
Use our due date calculator, which also works from a scan or an IVF transfer date.
Does “24 weeks” mean 24 full weeks?
Yes. “24 weeks 3 days” means 24 full weeks have passed and you are in your 25th week. The calculator uses completed weeks, like doctors do.