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WHAT IS MY DUE DATE? — Your due date, also called the estimated date of delivery, is 280 days from the first day of your last menstrual period, if you have regular monthly periods. Most women deliver in the two weeks before or after their due date. Approximately 1 of every 25 pregnant women delivers exactly on their due date.
Due date calculator — If you have regular monthly menstrual cycles and know the first day of your last menstrual period, you can figure out when your baby is due (estimated due date) and how far along you are in your pregnancy now (estimated gestational age) by using an online calculator (calculator 1).
Ultrasound — If you do not have regular monthly menstrual cycles or do not know when your last menstrual period started, you will need to have an ultrasound examination to determine your due date. This examination should be done as soon as possible because ultrasounds done after the fifth month of pregnancy (22 weeks of gestation) are not reliable for predicting due dates.
An ultrasound is commonly done to confirm the due date even in women who know their last menstrual period and who have regular periods. Sometimes the due date predicted by your last menstrual period is different from that predicted by the ultrasound examination. If this happens, the health care provider will probably change your due date to the date predicted by the ultrasound examination. Ultrasound examination is more reliable than last menstrual period for predicting the due date.
WHY IS THE DUE DATE IMPORTANT? — Health care providers use the due date to figure out how far along you are in your pregnancy at each office or hospital visit. A lot of decisions are based on this information. For instance, deciding when to do certain laboratory tests or other tests on you or your baby and whether labor is starting too early or too late.
