First day of last period, Average cycle length, Today
Pregnancy Due Date Calculator
Estimate your due date from the first day of your last menstrual period.
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How this calculator works
The standard estimate (Naegele's rule) counts 280 days, or 40 weeks, from the first day of the last period, assuming a 28-day cycle. The calculator shifts the date for longer or shorter cycles. Only about 1 in 20 babies arrives on the estimated date; an early ultrasound gives the most reliable dating.
Formula
- Due date = first day of last period + 280 days + (cycle length − 28 days)
- Estimated conception = due date − 266 days
- Second trimester starts at 14 weeks 0 days; third trimester at 28 weeks 0 days
Worked example
Last period started January 1, 2026
- First day of last period
- 2026-01-01
- Average cycle length
- 28
- Today
- 2026-03-12
ResultThursday, October 8, 2026
Estimated due date, 40 weeks from your last period adjusted for a 28-day cycle.
Things to keep in mind
- Your clinician may change the date after a dating ultrasound.
- For general information only. It is not medical advice; talk to a health professional about your own situation.
Frequently asked questions
How is a due date calculated?
Add 280 days (40 weeks) to the first day of your last menstrual period. For cycles longer or shorter than 28 days, add or subtract the difference.
How accurate is the due date?
It is an estimate. Most babies are born within two weeks either side of it, and only around 4-5% arrive on the exact day.
When does each trimester start?
The first trimester runs to 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days until birth.
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Results are estimates based on the values you enter and the published method shown above. They do not replace professional medical, financial or legal advice. Method reviewed October 7, 2026. About this site