To estimate the date of conception, a woman counts 11 to 21 days from the first day of the last period, according to American Pregnancy Association. While this is a large range of days, it is an average of conception for women. Some women, such as those who track their ovulation or have fertility treatments, can narrow down the date of conception more precisely.
Women can also estimate the gestational age, which is done by looking at the first date of her last period, instead of the date of conception, says the American Pregnancy Association. This is only an accurate method if she has a regular cycle, where her period lasts about the same amount of time each month. Another way gestational age is estimated is with an ultrasound. This can be done as soon as five to six weeks after the start of her last period. It is more accurate to do early on in pregnancy.
She can calculate her due date also by looking at the last menstrual period, notes the American Pregnancy Association. She should count 40 weeks from the first date of her previous menstrual period. Approximately 5 percent of babies are born according to the estimated due date. These methods only work well for women with relatively regular cycles. If she has an irregular menstrual period, the estimate might be slightly off.