You need one piece of information to calculate your baby's estimated due date: the date of the first day of your last menstrual period (also known as your LMP). An average pregnancy lasts 40 weeks from that date, although it's common for babies to arrive any time after the 38th week. According to the American Congress of Obstetricians and Gynecologists, only about 5 percent of babies are born on their due date, so keep in mind that your baby's estimated due date is truly just an estimate.
One quick method for calculating your estimated due date is to add 7 days to the date of the first day of your LMP. Jump ahead one year, and subtract 3 months.
If your cycles are irregular, your doctor may schedule you for an early ultrasound, in addition to the physical exam, to verify how far along you are in your pregnancy.