Conception Date Calculator
When conception most likely happened, from a due date, last period or birth date.
Good to know. Pregnancy is counted from the last period, so at conception a pregnancy is already two weeks along.
Not medical advice. Only your midwife or doctor can date a pregnancy.
How it works
In plain words
A pregnancy lasts about 266 days from conception, so conception is about 266 days (38 weeks) before the due date. From the last period, ovulation and conception are about 14 days before the next period would have started.
The formula
conception ≈ due date − 266 days conception ≈ last period + cycle length − 14 days
Worked example
With a due date of 25 December 2026, conception most likely happened 266 days earlier, on Friday, April 3, 2026.
Sources
- ACOG Committee Opinion 700, Methods for Estimating the Due Date, 2017 280 days from the last period, and the accuracy of first-trimester ultrasound.
- Wilcox, Weinberg and Baird, Timing of sexual intercourse in relation to ovulation, New England Journal of Medicine, 1995 The six-day fertile window.
Sources checked September 30, 2026. 3 automated tests check its answers on every change.
About the conception date calculator
A due date is counted as 40 weeks from the first day of the last period, not from conception. Ovulation, and so conception, usually happens about two weeks after that period starts, so conception is about 38 weeks, or 266 days, before the due date. This calculator works back to that date from a due date, the last period or a birth date.
Conception can’t be pinned to a single day from dates alone. Sperm can survive up to five days, so intercourse up to five days before ovulation can lead to pregnancy, and ovulation itself varies from cycle to cycle. The window shown covers the days around the estimate when conception was most likely.
From a birth date, the uncertainty is larger, because babies are born anywhere from 37 to 42 weeks and still count as full term. A baby born early or late shifts the estimate by the same amount, so the window spans about five weeks.
A dating ultrasound in the first trimester is the most accurate way to date a pregnancy, to within about five to seven days. If you have one, use its due date here. These estimates can’t establish paternity; only a DNA test can.
Questions people ask
How do I calculate my conception date?
Count back 266 days (38 weeks) from your due date, or add about 14 days to the first day of your last period in a 28-day cycle.
How accurate is a conception date calculator?
It gives the most likely date, give or take several days. Ovulation timing varies, and sperm can survive up to five days.
Can a conception date prove who the father is?
No. The possible window is too wide, and cycles vary. Only a DNA test can establish paternity.