A MULTISTATE MODEL OF FECUNDABILITY AND STERILITY

This paper develops a multistate hazards model for estimating fecundab ility and sterility from data on waiting times to conception. Importan t features of the model include separate sterile and nonsterile states , a distinction between preexisting sterility and sterility that begin s after initiation of exposure, and log-normally distributed fecundabi lity among nonsterile couples. Application of the model to data on fir st birth intervals from Taiwan, Sri Lanka, and the Amish shows that he terogeneity in fecundability is statistically significant at most ages , but that preexisting sterility and new sterility are unimportant bef ore age 40. These results suggest that sterility may not be an importa nt determinant of natural fertility until later reproductive ages.