Naegele's rule

Naegele’s rule is the standard obstetric formula for estimating a pregnancy due date: EDD = LMP + 280 days, where LMP is the first day of the last menstrual period and EDD is the estimated date of delivery.

Devised by German obstetrician Franz Karl Naegele in 1830 and used clinically ever since. The original mnemonic: add 7 days to the LMP, subtract 3 months, add 1 year — which arithmetic resolves to LMP + 280 days.

Assumptions baked into the rule: a 28-day menstrual cycle, ovulation on cycle day 14, conception within 24 hours of ovulation. Real-world variability is substantial — cycle lengths range from 21 to 35+ days, and the standard deviation of actual delivery dates around the predicted EDD is about 13 days. Only ~5% of babies arrive exactly on their predicted due date.

First-trimester ultrasound dating supersedes Naegele’s rule whenever available — it measures the embryo directly and is far more accurate than menstrual dating. Our due date calculator implements Naegele’s rule with both LMP and conception-date inputs.