Conception date calculator: when conception most likely occurred

A conception calculator can narrow the date to a window, not to a day

Fertilization occurs within about a day of ovulation, because an egg survives for about 12 to 24 hours after it is released. Sperm, however, can live in the body for as long as five days (ACOG, Trying to get pregnant? Here’s when to have sex). Intercourse in the five days before ovulation can therefore lead to pregnancy as well as intercourse on the day itself. For that reason the estimated date of conception is given as a six-day window ending on the estimated day of ovulation, the interval in which every pregnancy began in the study by Wilcox, Weinberg and Baird (New England Journal of Medicine, 1995).

Conception date based on due date: 266 days earlier

Naegele's rule places ovulation, and so conception, 266 days (38 weeks) before the due date (StatPearls, Naegele’s rule). That is the 280-day count from the last period less the 14 days to ovulation that the rule assumes. A due date set at a first-trimester dating scan gives a more reliable estimate than one calculated from a last period, because the scan measures the pregnancy directly (ACOG Committee Opinion 700, 2017, reaffirmed 2022).

Working back from a birthday is less accurate than working back from a due date

About 5 in 100 babies are born on their due date, and about 66 in 100 within a week of it (Khambalia and colleagues, 2013, in 18,708 single births after spontaneous labour in one Australian health district). A conception date calculated from a date of birth can therefore be wrong by a week or more. Where the due date is known, it gives the better estimate.

Dating can suggest a window but cannot establish paternity

A conception window spans several days, and the dating methods behind it have their own margin of error. Dating alone cannot show who the biological father is. Where paternity is in question, DNA testing is the method that can establish it.