Calculators5 min readUpdated: 2026-09-15
What Is an Estimated Due Date (EDD)? Naegele's Rule & Gestational Age
Definition
The Estimated Due Date (EDD) is the projected calendar date when a pregnant person is expected to deliver a full-term infant, conventionally set at 280 days from the LMP.
Detailed Technical Breakdown
Established by German obstetrician Franz Naegele, Naegele's Rule calculates the estimated delivery date by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP), adjusted for cycle lengths different from 28 days. Because biological conception occurs at ovulation (approximately 14 days after LMP), the fetus's true embryonic age is approximately two weeks younger than clinical gestational age.
Key Technical Specifications
- Formula: EDD = LMP + 280 days + (Cycle Length - 28 days).
- Standard duration: 40 completed gestational weeks (280 days).
- Full term delivery window: 37 weeks, 0 days through 40 weeks, 6 days.
- Early ultrasound Crown-Rump Length (CRL) supersedes LMP if difference exceeds 7 days.
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