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Calculating your due date
Q: I just had a positive pregnancy test. How can I find out when my due date?

A: A full term pregnancy lasts about 280 days counting from the first day of your last menstrual period. If you have regular menstrual cycles, you can calculate your due date if you know the first day of your last menstrual cycle. Once you recall this date, add one week and subtract three months.

For example, if your last menstrual period was January 14, your due date would be October 21. (The 14th day of the month plus one week is the 21st day. The month that occurs three months prior to January is October.)

If your menstrual cycles have been irregular or you conceived while on the contraceptive pill, you'll need to consult with your healthcare provider to determine your due date. A pelvic exam in the first trimester may be helpful and an early ultrasound may be needed to estimate the date.

Keep in mind that few babies actually deliver on the estimated due date. Most are born somewhere in the two weeks before or after this date.
Laura E. Stachel M.D. Obstetrician & Gynecologist