Why intercourse dates give a range
Conception doesn’t always happen on the day you have sex. Sperm can survive in the reproductive tract for up to about five days, waiting for an egg to be released. If ovulation happens two or three days after sex, that is when conception occurs. So a single intercourse date can lead to conception on that day or any of the next five or so.
The Pregnancy Due Date Predictor takes this into account. Enter one or more dates from the same cycle and it calculates the possible conception window and the matching due date range — 266 days after each end of the window — with the middle as the best single estimate.
Conception window = first date → last date + 5 days
Due date range = conception window + 266 days
Narrowing the range
- Your last period: combined with your cycle length, it is the standard way to date a pregnancy — use the Pregnancy Calculator.
- Ovulation tracking: if you know your ovulation day from tests or temperature charting, use the Due Date Calculator by Conception.
- A dating scan: an ultrasound in the first trimester dates the pregnancy to within about a week.
When to test
Because conception may happen a few days after sex and implantation takes another 6–12 days, a home test is most reliable about three weeks after the last time you had unprotected sex, or from the day your period is due. The calculator shows that date. If the result is negative and your period still hasn’t come, test again a few days later.