conceptional age, in fetal development the number of weeks since conception. Because the exact time of conception is difficult to determine except in some cases of the use of artificial reproductive technology, conceptional age is assumed to be 2 weeks less than gestational age. Gestational age is in turn the number of weeks that have elapsed between the first day of the last normal menstrual period in an idealized 28-day menstrual cycle and the date of delivery. See also estimated date of delivery, Naegele’s rule.
