How is your pregnancy due date calculated?

To find your pregnancy due date, your healthcare professional starts with the date your last menstrual period began, adds seven days, counts back three months and puts that date in the following year. Or your healthcare team may estimate your due date by simply counting ahead 40 weeks from the start of your last period. Typically, a baby is born about 38 weeks from conception. But pregnancy is measured using a 40-week time frame, starting with the first day of the last period.

Estimating a pregnancy due date is an important part of prenatal care. But it's not an exact science. Many babies are born a week or two before or after their due dates.