What is Naegele's rule?

Naegele's rule is a method of estimating the due date by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). Developed by 19th-century German obstetrician Franz Karl Naegele, it is the standard formula used by obstetricians worldwide. It is based on an average pregnancy length of 280 days and assumes a 28-day menstrual cycle with ovulation occurring 14 days after the start of the period. About 5% of births occur exactly on the EDD, about 65% within ±1 week, and about 90% within ±2 weeks. The estimate is less accurate for cycles that are not 28 days or for IVF pregnancies, so ultrasound measurements from your obstetrician are more precise.

How are ovulation and the fertile window calculated?

This site assumes a 14-day luteal phase. Ovulation is estimated as next period minus 14 days, and the fertile window covers 5 days before to 1 day after ovulation (7 days). Sperm can survive up to 5 days, so intercourse a few days before ovulation may also result in pregnancy. The 14-day luteal phase is a statistical average — actual length varies from 8 to 16 days. For more accurate ovulation tracking, combine with basal body temperature (BBT), LH tests, and cervical mucus observation.

Due Date Calculator — ACOG Naegele Formula

The formula

This calculator uses Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP) to estimate the due date (EDD). It has been the standard obstetric formula since the 19th century and is recommended by ACOG as the first-line estimation method.

Limitations

Naegele's rule relies on the assumptions below. When your situation differs, the error grows — confirm your EDD with an early ultrasound at your obstetrician.

Assumes a 28-day menstrual cycle. Longer cycles push the real EDD later; shorter cycles, earlier.
Assumes ovulation occurs 14 days after the start of the period. Luteal-phase length varies between individuals.
Only about 5% of babies are born on the EDD itself; roughly 65% within ±1 week and 90% within ±2 weeks.
For IVF or irregular cycles the error is larger — ultrasound measurement (CRL) is more accurate.

Frequently asked questions

My clinic gave a different EDD — which one should I trust?

Trust the clinic. Obstetricians combine Naegele's rule with an early-pregnancy ultrasound (weeks 8–13) that measures fetal crown-rump length (CRL), which is more accurate than LMP alone. This calculator uses LMP only.

My cycle is not 28 days — is the result still accurate?

The further your cycle deviates from 28 days, the larger the error. A 35-day cycle, for example, can push the real EDD about a week later than what this calculator shows. We plan to add a cycle-length input in a future update; for now, prefer an early-ultrasound EDD if you have one.

I conceived via IVF — how should I calculate?

For IVF pregnancies LMP is unreliable. Use our dedicated IVF Due Date Calculator instead — it counts from the embryo transfer date (day-3 or day-5 embryo) and is more accurate.

Counting down the days until you meet your baby