How it works
Conception usually happens around ovulation, about 14 days after the first day of your last period in a 28-day cycle.
From your due date
We count back 266 days (38 weeks) from your due date. That is the most likely day of conception.
From your last period
We use your usual cycle length. Ovulation is about 14 days before your next period would start, so a longer cycle moves the date later and a shorter cycle moves it earlier.
The 6-day window
Sperm can live for several days, so the sex that led to the pregnancy most likely happened in the 6 days ending on ovulation day. We show that window.
Why the date is never exact
- It is hard to know exactly when ovulation happens, and the day can change from cycle to cycle.
- Sperm can live up to 7 days, so sex a few days before ovulation can still lead to a pregnancy.
- An early ultrasound is the most accurate way to date a pregnancy.
This is an estimate. The real day may be different. It cannot prove who the father is.
Questions people ask
Can I know the exact day I got pregnant?
Usually not. It is hard to know exactly when ovulation happens, and sperm can live up to 7 days (NHS). An early ultrasound gives the most accurate pregnancy dates.
How many weeks pregnant am I at conception?
About 2 weeks. Pregnancy weeks are counted from the first day of your last period, and in a 28-day cycle conception happens about 2 weeks later.
How do I find my conception date from my due date?
Subtract 266 days (38 weeks) from your due date. That is the most likely day. Example: a due date of 14 May 2027 gives about 21 August 2026.
Which days could the sex have happened?
Most likely in the 6 days ending on ovulation day. Sperm can live up to 7 days (NHS), so a little earlier is possible too.
Can I use my ultrasound date?
Yes. Enter the due date your scan gave you. A scan early in pregnancy is the most accurate way to date it (ACOG).
Can the conception date show who the father is?
No. The dates are estimates and can be off by several days or more. Only a paternity test can answer this question.
Why is my pregnancy counted from my last period, not from conception?
Because the first day of the last period is a date most people know. Doctors count pregnancy weeks from it, so conception happens at about “2 weeks pregnant”.
Read the full guide: Conception date calculator
Sources
- ACOG Committee Opinion 700: Methods for Estimating the Due Date · checked October 24, 2026
- NHS: Periods and fertility in the menstrual cycle · checked October 26, 2026