The estimated date of delivery (EDD), also known as the estimated due date or expected date of confinement (the latter now considered to be an archaic term), is a term describing the date upon which a pregnant woman is predicted to give birth. While childbirth on average occurs at a gestational age of 280 days (40 weeks), there is considerable variation on this average, and the actual conception date may only be estimated, it may be more helpful to describe this date as a range rather than a single "due date". The EDD is typically determined based on the first day of a woman's last menstruation. See Naegele's rule. This class represents an Observation Clinical Statement wherein the Estimated Due Date has been Evaluated, which can be used in a variety of other models as if it were a datatype.