The estimated due date (EDD or EDC) is the date that spontaneous onset of labor is expected to occur. The accuracy of the due date will depend on the method used to calculate the date. With the exception of pregnancies conceived through in vitro fertilization the most accurate method for establishing a due date is generally an ultrasound performed during the first trimester.

The due date may be estimated by adding 280 days ( 9 months and 7 days) to the first day of the last menstrual period (LMP). This is the method used by "pregnancy wheels". The accuracy of the EDD derived by this method depends on accurate recall by the mother of her last LMP, assumes regular 28 day cycles, and that conception occurs on day 14 of the cycle with no variability.

Ultrasound uses the size of the fetus to determine the gestational age (the time elapsed since the the first day of the last menstrual period). Since most babies grow at a similar rate during the first trimester the due date can be calculated to within +/- 3 to 5 days using this method. As the baby grows the size of the baby becomes a much less accurate method of determining the gestational age, because factors such as the parent's height and maternal diabetes cause babies to grow at different rates. By  20 weeks' the  error is +/- 7 to 10 days which is still acceptable for establishing a due date. However, the error becomes progressively greater as the baby grows so that by 32 weeks the error has increased +/- 3 weeks making the measurement too inaccurate for establishing a due date.

In cases where the date of conception is known precisely, such as with in vitro fertilization, the EDD is calculated by adding 266 days to the date of conception.