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Pregnancy Due Date Calculator (Naegele's Rule)

Estimate a pregnancy due date and trimester milestones from the last menstrual period, adjusted for average cycle length.

Estimated due date
2026-10-08
Current gestational age
35 weeks, 2 days
Days until due date
33
Second trimester begins
2026-04-02
Third trimester begins
2026-07-09

How it works

This uses Naegele's rule (1812), the standard clinical estimate: due date = last menstrual period (LMP) + 280 days (40 weeks), based on a textbook 28-day cycle with ovulation on day 14. For a longer or shorter average cycle, the estimate shifts by the difference from 28 days, since a longer cycle means ovulation — and so conception — likely happened correspondingly later relative to the LMP.

Current gestational age is measured the same way pregnancies are dated clinically: as time elapsed since the LMP, not since conception, which is why 'weeks pregnant' is already a few weeks ahead of when conception actually occurred.

The second and third trimester start dates use the conventional 13-week and 27-week gestational-age milestones, counted from the LMP.

FAQ

Why measure from the last period instead of the actual conception date?

The exact date of conception usually isn't known, but the start of the last period typically is, so obstetric dating has used LMP as the standard reference point since Naegele's rule was first published. An ultrasound taken early in pregnancy can refine the estimate, but this calculator only knows what a plain LMP-based formula can compute.

How accurate is this estimate?

Naegele's rule is a widely used starting estimate, not a guarantee — only a small share of births happen on the exact calculated due date, and actual cycle and ovulation timing varies by individual even after the cycle-length adjustment. It's meant as a planning reference, not a medical diagnosis.

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