Question 6: Based on data from the National Center for Health Statistics, N. Wetzel used the Normal distribution to model the length of gestation for pregnant U.S. women (Chance, 2001). Gestation has a mean length of 280 days with a standard deviation of 20 days. 2 (a) Find the probability that the length of gestation is between 275.5 and 276.5 days. (This estimate is the probability that a woman has her baby four days earlier than the Solution (a) P(275.5.