How it works
This uses Naegele's rule, the method most doctors start with: the due date is 280 days (40 weeks) after the first day of your last menstrual period. If your cycle is longer or shorter than 28 days, the date moves by the difference.
Only about 1 in 20 babies arrive on the due date, so think of it as the middle of a window. Your doctor or midwife may adjust it after an early ultrasound.
Worked example
Last period started March 1, 2026, 28-day cycle: due date = March 1 + 280 days = December 6, 2026. On October 21, 2026 that's 33 weeks, 3 days, in the third trimester, with 46 days to go.
Frequently asked questions
How is the due date calculated?
Add 280 days to the first day of your last period, then adjust for cycle length (Naegele's rule).
How many weeks pregnant am I?
Pregnancy weeks count from the first day of your last period, so at conception you're already about 2 weeks along.
When does each trimester start?
First: weeks 1–13. Second: weeks 14–27. Third: week 28 to birth.
Is this medical advice?
No. It's an estimate. Your healthcare provider will confirm your due date.