Work backward from your due date, last period or ultrasound to estimate when conception most likely happened.
Conception happens when a sperm fertilizes an egg, usually within a day of ovulation. Because ovulation is hard to pin down, the date is worked out from something you do know: your due date, the first day of your last period, or an ultrasound that gave the age of the pregnancy. In a typical 28 day cycle, ovulation falls around day 14, so conception is about two weeks after the last period began and about 38 weeks, or 266 days, before the due date.
No method can name the exact day. The calculator shows a most probable range of about five days around the estimate and a wider range of possible dates, since ovulation can come earlier or later than expected. It also shows the dates of intercourse that could have led to the pregnancy. Sperm can live in the body for three to five days, so intercourse up to five days before conception can result in pregnancy.
If you start from your last period, the length of your usual cycle matters. Ovulation tends to happen about 14 days before the next period, so with a 35 day cycle, conception is more likely around day 21 than day 14. For an ultrasound, the calculator takes the date and the length of the pregnancy found by the scan, works back to the start of the last period, and then follows the same steps. Early scans are the most accurate way to date a pregnancy.
A due date is set 280 days after the first day of the last period, and conception is about 14 days after that. So conception is estimated at 266 days before the due date.
Ovulation does not always happen on the same day of the cycle, and sperm can survive for several days. The calculator therefore gives a most probable range and a wider range of possible dates.
Sperm can survive for three to five days in the female body, which is why intercourse before ovulation can still lead to pregnancy.
Yes. Ovulation usually happens about 14 days before the next period starts. A longer cycle means later ovulation and a later conception date.
No. It only estimates dates and cannot confirm paternity. Only a medical or genetic test can do that.
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