How Due Dates Are Calculated (Naegele's Rule)

Guides. Updated October 11, 2026. Calculated with the standard pregnancy dating method used by ACOG and the NHS.

Most due dates are calculated with a method known as Naegele's rule: take the first day of the last menstrual period, add one year, subtract three months and add seven days. That's the same as adding 280 days, or 40 weeks.

Why the last period?

The start of a period is a date most people know, while the exact day of conception usually isn't. Counting from the period adds roughly two weeks before conception, which is why pregnancy is said to last 40 weeks rather than 38.

Adjusting for cycle length

The rule assumes a 28-day cycle with ovulation on day 14. With a longer cycle, ovulation and the due date move later by the same number of days; with a shorter cycle, earlier.

Ultrasound dating

A first-trimester ultrasound that measures the baby from head to bottom is the most accurate way to date a pregnancy. If it differs from the period-based date by more than about a week, providers usually use the scan.

A date, not a deadline

Only a small share of babies arrive on their due date. Full term is from 39 weeks to 40 weeks and 6 days, and birth any time from 37 to 42 weeks is common.

This article is general information, not medical advice. Talk to a doctor or midwife about your own pregnancy or fertility.