How can you calculate when you got pregnant?

To calculate the date of conception, a woman with regular menstrual periods counts 11 to 21 days from the first day of her last cycle, according to the American Pregnancy Association. Since most women do not know the exact date they conceived, the conception date is just an estimate.

Women calculate their due dates based on their last menstrual period, advises the American Pregnancy Association. The estimated due date is 40 weeks following the first day of the last menstrual cycle. This date is just an estimate, since only around 5 percent of all babies are born on their estimated due dates.