How to calculate conception from a birthday
Count back 266 days
A pregnancy lasts about 266 days from conception to the due date. So if you were born on your due date, subtract 266 days from your birthday.
Example: born on 14 November 2000 → most likely conceived around 22 February 2000.
If the baby came early or late
Take the week of pregnancy at birth, change it into days and subtract 14. Then count back that many days. Example: born on 14 November 2000 at 38 weeks → 266 − 14 = 252 days → most likely conceived around 7 March 2000.
The possible range
Term babies are born between 37 and 42 weeks, so conception was between 245 and 280 days before the birthday. For 14 November 2000 that is 8 February to 14 March 2000. Try it with your own birthday.
Sources
- ACOG Committee Opinion 579: Definition of Term Pregnancy · checked October 24, 2026
- ACOG Committee Opinion 700: Methods for Estimating the Due Date · checked October 24, 2026