What if my cycle isn't 28 days?

The estimate will be less precise for shorter or longer cycles, since it assumes an average cycle length — a healthcare provider can adjust the estimate using ultrasound dating instead.

How reliable is a due date estimate in general?

Only a small percentage of babies arrive exactly on the estimated date — it's better understood as the center of a normal range than a precise prediction.

Who created Naegele's rule and when?

German obstetrician Franz Naegele formalized it in his 1812 textbook, based on the observation that pregnancies last roughly 10 lunar months from the last menstrual period — it's remained the standard for over two centuries.

Does the rule apply equally to everyone?

A 2021 medical review found factors like ethnicity, height, cycle length, parity, and maternal weight all measurably affect actual gestational length, so Naegele's rule is best treated as a population-average estimate rather than a precise figure for any individual.

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Health & Fitness

Pregnancy Calculator

Estimated due date, week, and trimester.

🔒 Runs entirely in your browser — nothing here is ever uploaded

About the Pregnancy Calculator

Estimates a due date, current gestational week, and trimester from the first day of your last menstrual period, using Naegele's rule — a standard obstetric estimation method that assumes a 280-day (40-week) pregnancy from LMP.

How to use it

Enter the First day of last menstrual period.
Read your estimated due date, current week and day, and trimester in the result panel.

Formula

Due date = LMP + 280 days
Current week = (Today − LMP) ÷ 7, rounded down

Naegele's rule, formalized by German obstetrician Franz Naegele in his 1812 textbook and still the global standard estimation method, though a 2021 review found actual gestational length varies by ethnicity, height, cycle length, parity, and maternal weight.

Worked example

An LMP of January 1 gives an estimated due date around October 8, 280 days later.

Recommendations

• This is a statistical estimate based on average cycle length — actual delivery commonly falls within a couple of weeks of the estimate in either direction.
• An early ultrasound dating scan is generally considered more accurate than an LMP-based calculation, especially with irregular cycles.
• Trimesters are commonly defined as weeks 1–13, 14–27, and 28–40, though exact boundaries vary slightly by source.

Frequently asked questions

It's based on a 28-day average menstrual cycle with ovulation around day 14, giving roughly 40 weeks from the last period to a full-term birth — the standard assumption behind Naegele's rule.

Related tools

Ovulation Calculator

Age Calculator

Date Calculator

Sources

Lawson GW — Naegele's rule and the length of pregnancy, a review (Aust N Z J Obstet Gynaecol, 2021) — accessed 2026-08-19

Disclaimer

Educational estimate, not medical advice. Consult a qualified clinician before making health decisions.