Working backwards from due date to conception
Most pregnancy dating works forward from the last menstrual period to estimate a due date. This calculator runs that process in reverse — if you already know (or have been given) a due date, it estimates when conception likely occurred, based on the same standard 280-day pregnancy length assumption.
Because ovulation typically occurs around two weeks into a standard cycle, conception date is estimated as 266 days before the due date (280 days minus the roughly 14-day gap between last period and ovulation) — slightly later than the last menstrual period date used in due-date calculations.
Why this is an estimate, not a fact
Actual conception timing depends on individual cycle length and ovulation timing, both of which vary. This calculator assumes a standard 28-day cycle; anyone with a notably longer or shorter cycle conceived at a correspondingly shifted time relative to this estimate.