What has caused the steep decline in male fertility, asks Steve Jones, science writer for the UK’s Independent. The latest research, he reports, is that environmental chemicals are much less important than the environment of the womb.

Male infertility is a medical puzzle (as well as a great commercial opportunity for IVF clinics). About 20 years ago a Danish scientist, Niels Skakkebaek, of the University of Copenhagen, demonstrated that sperm counts had fallen by nearly 50% since the 1940s. At that time, sperm counts were well above 100m sperm per millilitre, but by 1990 they had dropped to 60m per ml. Other studies have found that between 15 and 20 percent of young men have sperm counts of less than 20m per ml, which is technically defined as abnormal.

Environmental chemicals may play a role, but not a decisive one, says Professor Richard Sharpe, of the UK’s Medical Research Council. "Public concern about the adverse effects of environmental chemicals on spermatogenesis in adult men are, in general, not supported by the available data for humans. Where adverse effects of environmental chemicals have been shown, they are usually in an occupational setting rather than applying to the general population," he says.

More and more evidence points to the womb. The number of sperm depend on the number of Sertoli cells that develop in the male foetus, so anything that interferes with the formation of Sertoli cells in a mother's womb will affect sperm production many years later. "Maternal-lifestyle factors in pregnancy can have quite substantial effects on sperm counts in sons in adulthood," says Professor Sharpe.

Curiously, Connor never once mentions the contraceptive pill, even to discount it, although oestrogen disrupts male development. ~ Independent, Apr 26