Work backwards from your due date to estimate when conception likely occurred, or forward from your last period date. Shows gestational age, trimester, and conception window.
Due date (Naegele's rule) = LMP + 280 days (or equivalently: LMP + 9 months + 7 days). Estimated conception = LMP + (cycle length − 14) days ± 2–3 days. Reverse: LMP = Due date − 280 days; Conception window = Due date − 266 days (±3 days).
The 280-day (40-week) gestational period is measured from LMP, not from conception. Conception occurs approximately 14 days after LMP (for a 28-day cycle), so actual fertilisation-to-birth duration is approximately 266 days (38 weeks). This distinction matters when working backwards from a due date — the calculator subtracts 266 days from the due date to find the estimated conception date, and 280 days to find LMP.
Worked example: due date 15 January. LMP estimate = 15 Jan − 280 days = 9 April (prior year). Estimated ovulation = 9 April + 14 days = 23 April. Conception window = 20 April – 25 April. Current gestational age (from today's date) = number of days from estimated LMP ÷ 7 = weeks + days.