Longitudinal data from Taiwan in the period 1960--1973 are used in an age-dependent non-Markovian Stochastic model of human reproduction to evaluate the effects of changing contraceptive technology and extended use patterns on fertility. The fertility experience of women claiming to be limiters up to the first unwanted pregnancy following acceptance of an IUD is considered. It is found that improved contraceptive technology, and expecially changed switching patterns among methods, account for as much as a 50% decline in first pregnancy probabilities over period studied.