Estimated Due Date (EDD)

Calculate the Estimated Date of Delivery using Naegele's rule based on the Last Menstrual Period.

JavaScript is disabled. To calculate manually using Naegele's rule: Add 7 days to the first day of your LMP, then subtract 3 months.

First day of Last Menstrual Period (LMP)

Average Cycle Length (Days)

Naegele's rule assumes a 28-day cycle. Adjust if your cycle is consistently longer or shorter.

Understanding Naegele's Rule

Naegele's rule is the standard obstetrical heuristic for calculating an Estimated Date of Delivery (EDD). It adds exactly 280 days (40 weeks) to the first day of the last menstrual period (LMP).

Because conception typically occurs around ovulation (day 14 of a 28-day cycle), the "gestational age" actually includes two weeks where the patient was not pregnant.

Clinical Accuracy

Only about 4-5% of babies are born on their exact EDD.
A normal, full-term pregnancy is defined as delivery between 37 weeks and 42 weeks of gestation.
First-trimester ultrasound (Crown-Rump Length measurement) is considered the most clinically accurate method for dating a pregnancy, superseding LMP if there is a discrepancy of more than 5-7 days.