expected date of delivery (EDD), the predicted date of a pregnant woman’s delivery. Pregnancy lasts approximately 266 days, or 38 weeks from the day of fertilization, but is considered clinically to last 280 days, or 40 weeks, or 10 lunar months, or 9⅓ calendar months from the first day of the last menstrual period (LMP). The EDD is usually calculated on the basis of 9⅓ calendar months, but if a woman is certain that coitus occurred only once during the month and if she knows the date on which it occurred, the EDD may be calculated as 38 weeks from that date. In the absence of a special calendar or device for calculating the EDD, it is arrived at by counting back 3 months from the first day of the LMP and then adding 7 days and 1 year; thus, if the first day of a woman’s LMP was July 18, 2014, one counts back 3 months to April 18, 2014, then adds 7 days and 1 year to arrive at an EDD of April 25, 2015 (Nägele’s rule). Because calendar months differ in length, this calculation may give a date that is a few days more or less than 280 days from the first day of the LMP, but it provides a very close approximation, and a trivial error will not be of clinical significance because of the variability of the actual durations of normal pregnancies. The expectant mother is advised that the EDD is only an estimate and that the chances are that she will give birth within 2 weeks before or, more commonly, after the calculated date. Also called expected date of birth, expected date of confinement.
