Babies delivered at the standard 40-week mark measure an average of 20.16 inches long, according to BabyCenter.com. A baby born at 41 weeks gestation measures an average of 20.35 inches. A baby delivered between 38 and 39 weeks measures an average of between 19.61 inches and 19.96 inches. Of course, babies who are born prematurely tend to be smaller in length than babies who are born full-term.