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The waiting times to first births were collected in the population of Isle-aux-Coudres, Quebec. The monthly probabilities of fertility in first marriages of consanguineous and non-consanguineous couples were computed and fitted by a beta distribution in order to estimate the mean and variance of fecundability. Consanguineous couples were prone to less intra-uterine mortality in the first days following conception. Otherwise, they experienced a higher late intra-uterine mortality. At birth, non-consanguineous couples had had, however, more fetal deaths. It is also suggested that variation in the sexual behavior could account, more than intra-uterine mortality, for differences in natural fertility between populations.