Trends in sex-ratio, testicular cancer and male reproductive hazards: are they connected?

In the last few decades, the male proportion of newborn babies has been decreasing in several populations. The changes are very small and without practical importance per se, but the underlying biological mechanisms are not known. In the same period, testicular cancer incidence has increased, and there has been indications of decreasing sperm counts in men in several populations. The available knowledge on factors that influence the sex-ratio in humans supports the idea that an excess of girls in the offspring of a man may be an indicator of reproductive hazards. Data from a Danish case-control study show strong associations between testicular cancer, low fertility and a low M:F sex-ratio in the offspring. It is proposed as a hypothesis that there may exist common aetiological factors for testicular cancer, low fertility and low offspring sex-ratio, and that a search for the causal factors involved may focus on agents that can act prenatally to disrupt the normal development and differentiation of the male reproductive organs.