Calculate Your Due Date

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To calculate your due date, take the first day of your last menstrual period and add 280 days. This number includes the standard gestational period of 266 days plus 14 days. The added time is assuming that you ovulated exactly 14 days after the first day of your last menstrual period, which is not always true, and that the 14th day was the day which you conceived on.

Two other ways to arrive at the same date is to subtract 3 months form the first day of your last menstrual period, and add a week. Or add 40 weeks to the first day of your last menstrual period. A full term pregnancy is measured in lunar months, 10 months containing exactly 4 weeks each.

If you know the exact date of conception as a result of using ovulation prediction methods such as charting your basal body temperature, using an ovulation monitor or kit, or have had only one sexual encounter in the cycle, just add the standard 266 days of gestation to arrive at your due date.

If the exact date of conception is not known, and the first day of the first menstrual period can not be accurately recalled, your healthcare provider will determine the due date using other methods such as an ultrasound exam, size of the uterus, or the date when fetal heart tones can be heard. Using ultrasound to determine gestational age is most accurate early in the pregnancy and can be performed as early as 5 weeks from the first day of the last menstrual period, with the optimum time to perform this task is around the 8th to 18th week.

Due dates are only an estimation; even knowing the date of conception is no guarantee that your baby will arrive on the exact date they are due. Only 5% of babies are born on their due date, and it is in the range of normal for your baby to arrive 2 weeks before or after the due date.