When did I conceive?
Conception usually happens around ovulation, about two weeks after the first day of the last period and 266 days (38 weeks) before the due date. The calculator works back from your due date, your last period or an ultrasound to the most likely day of conception, and shows the days on which intercourse could have led to the pregnancy.
Example: with a due date of October 8, 2027, conception most likely happened around January 15, 2027, from intercourse between about January 10 and January 16.
Why it is a window, not a day
Sperm can survive in the body for up to five days, and an egg lives for about a day after ovulation. So intercourse in the five days before ovulation, or on the day after, can lead to pregnancy. Ovulation itself also varies from cycle to cycle, so the dates are estimates.
Conception date vs. pregnancy weeks
Weeks of pregnancy are counted from the last period, not from conception. At conception you are already about 2 weeks pregnant by that count, which is why a pregnancy lasts 40 weeks on paper but about 38 weeks from conception.
Frequently asked questions
Can a calculator tell who the father is?
No. Conception dates are estimates within several days, so they cannot settle paternity. Only a DNA test can.
Which date is most accurate?
An early ultrasound, done before about 14 weeks, dates a pregnancy more accurately than the last period, especially with irregular cycles.
Can I get pregnant during my period?
It is unlikely but possible with short cycles, because sperm can live up to five days and ovulation can come early.