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, spring 2001). gestation length has a mean of 280 days with a standard deviation of 20 days. (a) find the probability that gestation length is between 275.5 and 276.5 days. (this estimates the probability that a woman has her baby 4 days earlier than the ‘‘average’’ due date.)