Health Formula • Published on September 01, 2026 • Last updated September 01, 2026
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First day of the last menstrual period before conception
Standard full-term pregnancy duration (40 weeks) assumed from LMP
Average menstrual cycle length in days, used to adjust the estimated ovulation/conception date (default 28 days)
Naegele's Rule estimates a pregnancy due date by adding 280 days (40 weeks) to the first day of the last menstrual period, since gestational age is conventionally measured from LMP even though conception typically occurs about two weeks later. A cycle-length adjustment corrects for women whose cycles are longer or shorter than the standard 28-day assumption, since a longer cycle means ovulation (and conception) happens later.
The exact conception date is rarely known with certainty, whereas the LMP date is easy to identify, so obstetric convention measures gestational age from LMP as a reliable, consistent reference point.
Only about 5% of babies arrive exactly on their calculated due date — the estimate defines a likely delivery window (typically within 1-2 weeks either side), not a precise date, especially for LMP-based estimates versus early ultrasound dating.
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