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How is my due date calculated?
Q: How is my due date calculated?
A: Your due date is calculated by taking the first day of your last menstrual period, subtracting three months and adding seven days. As an example, if your last period started on June 6, you would take away three months—March— and add seven days—the 13th. Your due date would be March 13th.

The due date calculation is based upon a menstrual cycle of 28 days. If your cycle is shorter or longer, your due date will need to be tweaked slightly.