Some indirect relationships between contraception failures and offspring with a pathological condition are reviewed, and a causal connection between them is suggested. This hypothesis is supported by the decreasing incidence of Down syndrome (DS) in general in recent years, and the increase among the younger maternal age categories as well as the effect of birth order. A more than doubled incidence of DS children among young Catholic mothers and the fact that conceptions of children with DS are preceded by unusually long periods of abstinence are emphasised and discussed. These data offer circumstantial evidence that the conceptions that occur despite application of the so called "natural" family planning methods in particular are at high risk. The impact of reproductive biological failures ("nature") and of reproductive behaviour ("nurture") on the human gametes can be disentangled by thorough studies of the epidemiology of major and minor congenital anomalies.