What is Naegele's Rule?
Naegele's rule estimates the expected delivery date by adding 280 days (40 weeks) to the first day of the last menstrual period. It assumes a regular 28-day cycle with ovulation occurring on day 14 — an approximation that's convenient but imperfect for everyone.
How to calculate it
EDD = LMP + 280 days. This calculator also shows the current gestational age based on the same LMP date, in completed weeks and days.
When LMP dating is less reliable
- Irregular menstrual cycles
- Recent use of hormonal contraception
- Uncertain or unrecalled LMP date
In these situations, first-trimester ultrasound dating (using crown-rump length) is generally more accurate and is typically used to set the official EDD if it differs meaningfully from LMP-based dating.
References
Committee on Obstetric Practice, American College of Obstetricians and Gynecologists. Methods for estimating the due date. Obstet Gynecol. 2017.