Naegele's Rule and Estimated Due Date Calculation

The estimated due date (EDD) is a planning date, not a promise that birth will occur on that day. It corresponds to 40 weeks and 0 days of gestational age when calculated from the first day of the last menstrual period (LMP). Gestational age begins on the first day of menstrual bleeding, even though conception usually occurs about two weeks later. This convention gives clinicians a common reference point for prenatal screening, fetal growth assessment, and decisions affected by prematurity or post-term pregnancy. Naegele's rule assumes a 28-day menstrual cycle with ovulation around day 14. The calculation becomes less dependable when ovulation occurred later or earlier than expected, the cycle length is irregular, or the reported bleeding was not a typical menstrual period. The arithmetic may be correct while the date is still clinically wrong because the starting point was unreliable.