Health
An estimated due date from the first day of the last menstrual period, using Naegele's rule. The actual date of delivery varies from person to person.
Results update as you type. No calculate button. All calculations run locally in your browser.
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| Measure | Value |
|---|---|
| First day of last menstrual period | — |
| Estimated due date (LMP + 280 days) | — |
| Approximate conception date | — |
| Gestational age today | — |
| Trimester boundaries | — |
Gestational age today is computed live from the entered LMP to the current date, so it changes day by day; the due date itself does not change once the LMP is fixed.
Estimated due date = first day of last menstrual period + 280 days (40 weeks). This is Naegele’s rule. It assumes a regular 28-day cycle with ovulation on day 14, which is why the approximate conception date is shown as LMP + 14 days. Trimester boundaries shown are LMP + 14 weeks and LMP + 28 weeks.
Last menstrual period 2026-01-01: estimated due date = 2026-01-01 + 280 days = 2026-10-08. Approximate conception (28-day cycle assumption) is 2026-01-15. On the reference date 2026-09-10, gestational age is (2026-09-10 − 2026-01-01) = 252 days = 36 weeks 0 days, with 28 days (4 weeks 0 days) remaining to the estimated due date. Use Reset example above to reproduce the due date; gestational age today on the live page is computed from the actual current date.
Naegele’s rule is an estimate built on two assumptions that often do not hold exactly: a regular 28-day cycle and ovulation on day 14. Longer or irregular cycles, uncertain LMP dates, and ultrasound dating can all shift the estimate, and the actual date of delivery varies from person to person — only a minority of births fall exactly on the estimated date. This page is not a substitute for prenatal care records or clinical dating, and it makes no claim about the health of any pregnancy.
Adding 280 days (40 weeks) to the first day of the last menstrual period. It is the standard back-of-the-envelope dating rule, not a measurement.
Because cycles vary: ovulation is not always on day 14, cycle lengths differ, and the LMP date itself can be uncertain. Ultrasound dating often refines the estimate.
Then ovulation likely came later than day 14, and the true due date may be later than this estimate. The page does not adjust for cycle length — that adjustment belongs in clinical dating.
No. It is an estimate at the centre of a range; actual delivery dates vary widely around it.
No. This calculation runs locally in your browser and the values you enter are not sent to a CalcMintly database.
No. Use prenatal care records and clinical advice for medical decisions; this page only shows the arithmetic of Naegele's rule.
Results are estimates only and do not constitute professional or medical advice.