Pregnancy week calculator
Pick what you know, the first day of your last period or your due date, then the date you want the pregnancy age for.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter i know my, the date i know and pregnancy age on.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Pregnancy age in days = chosen date - first day of last period (or due date - 280). Weeks = days ÷ 7, remainder in days. First trimester to 13 weeks 6 days, second to 27 weeks 6 days, then third.
Example
Inputs: I know my = Last period start date, The date I know = 2026-06-01, Pregnancy age on = 2026-09-30. Result: Pregnancy age: 17 weeks 2 days; Completed weeks: 17; Trimester: Second trimester.
Limits and caveats
- Pregnancy is counted from the first day of the last period, about two weeks before conception.
- If your doctor dated the pregnancy by ultrasound, enter that due date instead.
Questions about this tool
Why count from the last period and not conception?
The period date is usually known while conception is hard to pin down, so pregnancy age runs about two weeks ahead of the baby’s actual age.
What does 17 weeks 2 days mean?
You have completed 17 weeks and 2 days, so you are in your 18th week.
When does each trimester start?
The second trimester starts at 14 completed weeks and the third at 28 completed weeks.