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Naegele’s Rule Due Date Calculator

Apply Naegele’s rule — LMP plus one year, minus three months, plus seven days — with an adjustment for cycles longer or shorter than 28 days.

First day of last period (LMP)

Average cycle length

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How it works

EDD = LMP + 1 year − 3 months + 7 days (+ cycle length − 28)

Naegele’s rule assumes a 28-day cycle with ovulation on day 14, giving 280 days (40 weeks) from the last period. Longer cycles usually mean later ovulation, so the date is pushed back by the extra days; shorter cycles pull it forward. Early ultrasound measurements override the rule when they disagree by more than a week.

Worked example

LMP 1 July 2026 → 1 July 2027 → 1 April 2027 → 8 April 2027. With a 32-day cycle add 4 days: 12 April.

Frequently asked questions

How accurate is it?

About half of births fall within a week of the date; the 37–42 week window is the realistic target.

I do not know my LMP.

Use a dating ultrasound or, if you know the conception date, add 266 days to it.

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Educational estimate only — your midwife or doctor will confirm dates and advice for your pregnancy.