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.

Why 280 days, and how ultrasound refines it: 280 days = 40 weeks, the conventional length of human gestation counted from the LMP. The actual time from conception to delivery is closer to 266 days (38 weeks), but LMP is what patients can self-report and ultrasound was historically unavailable, so the 40-week convention stuck. First-trimester ultrasound dating measures the crown-rump length (CRL) of the embryo and compares it to a standard growth curve — accurate to ±5 days when performed between 8 and 13 weeks. After 14 weeks, dating accuracy degrades to ±7-10 days because growth velocity varies more between fetuses. ACOG’s 2017 committee opinion recommends ultrasound dating as the reference whenever it differs from menstrual dating by more than 5-7 days in the first trimester.

What 40 weeks actually means clinically: “full term” spans 37 weeks 0 days to 41 weeks 6 days under the modern ACOG definition (which subdivides further into early term, full term, late term, and post-term). The median actual delivery in spontaneous-onset pregnancies is closer to 40 weeks 1 day for nulliparous women and 40 weeks for parous women. The Naegele EDD is therefore better thought of as the centre of a roughly two-week-wide window in which a healthy term delivery is statistically likely, not as an appointment date. Convertitive’s due-date tool shows the EDD plus a ±2-week confidence band for context. Reference: ACOG — Methods for Estimating the Due Date.

Worked example

LMP = 1 January 2026. Add 7 days = 8 January. Subtract 3 months = 8 October 2025. Add 1 year = 8 October 2026. That’s the EDD. Cross-check by counting days: from 1 January to 8 October is 31 (Jan) − 1 + 28 (Feb) + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 8 = 280 days. ✓ For a person with a 32-day cycle (4 days longer than the 28-day assumption), ovulation likely occurred on cycle day ~18 rather than 14, so the EDD should shift 4 days later — to 12 October 2026. Most pregnancy apps and clinic systems implement only the basic 280-day rule; the cycle-length correction is something patients with irregular cycles must raise with their provider, because the unadjusted EDD can bias inductions for “post-dates” pregnancy that are actually only at 40+0 by the corrected count.

When and why it matters

The EDD anchors prenatal-care scheduling: nuchal-translucency screening (11-13 weeks), anatomy scan (18-22 weeks), glucose tolerance testing (24-28 weeks), Group B strep swab (35-37 weeks), and induction decisions for post-dates (typically at 41-42 weeks). An EDD off by a week shifts every appointment by a week and, more importantly, shifts the gestational-age threshold at which steroids are administered for fetal lung maturity (24-34 weeks) or at which a delivery is classified as preterm. Modern obstetric practice deliberately rebases the EDD to the earliest reliable ultrasound and locks it — changing it again later, especially in the third trimester, leads to inconsistent management. Reference: NICE NG201 — Antenatal care.