How do you calculate the weeks of pregnancy?

Calculate how many weeks of a pregnancy have passed by beginning with the first day of the woman's last menstruation and counting forward, explains BabyCenter. Women who have been tracking their ovulation and know their conception date may start there instead.

Many women are not sure when they conceived, so most doctors and online calculators use the first day of the woman's most recent period as the beginning of pregnancy, states BabyCenter. They determine the due date by counting ahead 40 weeks. If a woman is sure of her conception date, then she may count forward 38 weeks to determine her due date.