Controlling for Postpartum Amenorrhea and Heterogeneity in the Analysis of Fecundability Using Birth Interval Data: A Simulation Study with Application to Hutterite Reproductive Histories

Despite numerous studies, the level and age pattern of fecundability are still under discussion in the demographic, biologic, and medical literature. Previous analyses, however, have not fully taken into account two factors affecting estimated fecundability: postpartum amenorrhea and variations in fecundability among women. This paper used simulation analysis to calibrate a new fecundability model, and it showed that fecundability by age, heterogeneity in fecundability, and postpartum amenorrhea by age could be estimated simultaneously from all birth intervals. An empirical analysis of Hutterite birth histories showed the following: that fecundability declined almost linearly from age 20 to age 40; that the fecundability of a 35 year old was half the level of fecundability of a 25 year old woman; and that fecundability varied significantly across women. For an average Hutterite woman, the waiting time to next live-birth conception was 13.1 months at age 30 and 22.2 months at age 40. These intervals included 6.5 months of postpartum amenorrhea.