Back-calculate probable conception window for pregnancy timeline planning.
Overview
Conception-date calculators estimate when fertilization likely occurred based on known due date or menstrual dating.
Because ovulation timing varies, outputs should be interpreted as a probable window rather than one exact day.
Steps
Use due date, LMP date, or current gestational age.
Adjust ovulation assumption beyond default 28-day cycle.
Estimate ovulation from cycle timing rules.
Conception usually occurs near ovulation.
Include a small date range around estimated day.
Use early ultrasound and provider guidance for official dating.
Math
Conception Date ≈ EDD - 266 days; or ≈ LMP + (cycle length - 14) days
Pregnancy duration from conception is about 266 days on average, while ovulation is often estimated around 14 days before next period.
Worked cases
Conception ≈ EDD - 266 days
Estimated conception around March 29, 2026
Conception ≈ LMP + (30 - 14) = +16 days
Estimated conception around April 21, 2026
Fertile/conception window commonly spans several days
Likely window: roughly May 8 to May 12
Why use this
Questions