Formula & method
Naegele's rule: due date = first day of last menstrual period + 280 days (40 weeks). Week = completed weeks since LMP; trimester 1 is weeks 1–13, trimester 2 is 14–27, trimester 3 is 28–40.
Worked example
If your last period started 26 January 2026, the estimated due date is 2 November 2026. On 30 October 2026 you would be in week 40, trimester 3.
FAQ
- How accurate is the due date?
- It is an estimate: only a small share of births land exactly on it. Ultrasounds refine the estimate.
- What if I know the conception date?
- Add 266 days to the conception date instead of 280 days from LMP.
- Is this medical advice?
- No. It is a planning estimate. Your clinician's dates and advice take precedence.
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