Conception Date Calculator
About the Conception Date Calculator
This calculator estimates the date conception likely occurred by subtracting 266 days (38 weeks) from the due date, or by subtracting the gestational duration from the actual birth date. The fertile window that led to conception is also shown.
The Formula
Conception date = due date − 266 days (standard). Adjusting for cycle length: conception ≈ due date − (280 − cycle length + 14) days. From birth date: conception = birth date − (weeks pregnant × 7) days + 14 days (adding back ~2 weeks before ovulation).
Frequently Asked Questions
How accurate is the conception date estimate?
Conception can only occur during a 12-24 hour window after ovulation. Sperm can survive for up to 5 days, so intercourse on any of the 5 days before ovulation can result in conception. The estimate is most accurate with a known due date from early ultrasound.
What if my baby was born early or late?
If you know your baby's actual birth date and how many weeks pregnant you were at delivery (from your medical records), enter both for a more accurate conception estimate. The calculator accounts for the actual gestational duration.