A prospective study of the mother's longevity and of her completed family size has been conducted on the basis of historical demographic records. We show that 1 to 5 pregnancies is associated with the greatest longevity in weakly inbred women and 11 pregnancies is associated with greatest longevity in more inbred women. Taking into account maternal inbreeding, completely sterile women and those who had a large number of pregnancies have an equal mean longevity. It is suggested that repeated pregnancies produce a cumulative hormonal effect, the distribution of which, following family size, should be bell-shaped.