About due dates
This pregnancy due date calculator estimates when your baby is due from the first day of your last menstrual period, using Naegele rule, and tells you how many weeks along you are and which trimester you are in. It is an estimate, not medical advice. It runs in your browser; nothing is uploaded.
How Due Date works
The formula
Naegele rule: due date = first day of last period + 280 days (40 weeks). If your average cycle is longer or shorter than 28 days, the date is shifted by the difference. Weeks along = days since the last period ÷ 7.
Worked example
Last period began January 1 with a 28-day cycle: due date = January 1 + 280 days = October 8. On March 26 you would be 12 weeks along (84 days ÷ 7), which is the end of the first trimester.
Only about 1 baby in 20 arrives on the exact estimated date — most births happen within two weeks either side.
Common uses
- Estimate a pregnancy due date
- Find how many weeks pregnant you are
- Identify your trimester
- Plan from your last period date
- Adjust for a longer or shorter cycle
- Track gestational age
- Count down to your due date
- Share an estimate with family