The exact date of conception is typically estimated to be around 11 to 21 days after the first day of the last menstrual period for women with regular cycles. Conception usually happens on the day of ovulation when an egg is fertilized, and ovulation tends to occur about two weeks after the start of the last period. Because sperm can survive for up to 3-5 days inside the female reproductive tract, the conception window spans several days around ovulation. If the date of the last menstrual period or the due date is known, conception date can be estimated using these as references. Ultrasound measurements early in pregnancy can also help refine the estimate more accurately. To find out specifically "when did I conceive" for an individual, one would typically calculate based on:

• The first day of the last menstrual period plus 11 to 21 days, or
• The estimated due date minus 266 days (about 38 weeks),
• Or by using early pregnancy ultrasound measurements.

Without exact personal data such as the date of the last menstrual period, the due date, or early ultrasound dates, an exact conception date cannot be pinpointed but can be estimated within a range of days around ovulation about two weeks after the last period began.