How to Estimate a Pregnancy Due Date
By Naegele's rule, due date = first day of last period + 280 days (40 weeks). Equivalently, add 1 year, subtract 3 months, add 7 days. A last period of 2025-09-01 gives a due date around 2026-06-08.
Open the interactive Everyday Lifestyle tool →Key facts
| Naegele's rule | LMP + 280 days (40 weeks) |
|---|---|
| Shortcut | +1 year - 3 months + 7 days |
| Gestation | 40 weeks |
| LMP 2025-09-01 | due ~2026-06-08 |
Frequently asked questions
How is the due date calculated?
Add 280 days (40 weeks) to the first day of your last menstrual period (Naegele's rule).
How many weeks is a full-term pregnancy?
About 40 weeks from the last period; term is generally 37-42 weeks.
Is the due date exact?
No, it is an estimate. Only about 4% of births occur on the exact due date; a scan can refine it.
Related pages
The calculations on this page are estimates based on common general-purpose formulas. They are not medical advice -- consult a doctor or other qualified professional for decisions about your health.
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