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.
Another opportunity to determine the day of the baby's fertilization is to see when the baby's heart beat is visible on the ultrasound machine. This feat usually occurs at 6 weeks, which means that a woman who can hear the heartbeat of her baby needs only count back 6 weeks to find the exact date, reports BabyMed.
Ovulation is the period of time that happens "around day 14 of a menstrual cycle," writes WebMD, and it typically lasts for 5 to 7 days. Before ovulation begins, most women experience vaginal secretions. The ovulation period will vary from individual to individual, however, but most ovulation cycles follow these rules.