This is all over the news today. Thoughts?

The sperm count of men in the western world has fallen by more than half over a period of 40 years, according to an international study described by its authors as “an urgent wake-up call” about declining male health.

“Decreasing sperm count has been of great concern since it was first reported 25 years ago,” said senior author Shanna Swan of Icahn School of Medicine at Mount Sinai, New York. “This definitive study shows . . . that the decline is strong and continuing.”

The researchers, led by Hagai Levine of Hebrew University of Jerusalem, screened 7,500 sperm studies carried out worldwide between 1973 and 2011. They chose for meta-analysis 185 studies that met high standards of design and laboratory practice.

The results, published in Human Reproduction Update, show declines of 52.4 per cent in sperm concentration and 59.3 per cent in total sperm count among men in North America, Europe, Australia and New Zealand. Subjects were not selected according to their fertility status.

“When we restricted the analysis to studies after 1995, we found no sign that the decline is levelling off,” said Dr Levine. “These findings have wide implications for public health.”

Other fertility experts who were not involved in the project echoed the authors’ concern. “The extent of the decline in sperm counts in the western world revealed in this study is shocking,” said Professor Daniel Brison of Manchester university.

Professor Allan Pacey of Sheffield university, who has been sceptical about previous research showing declining sperm counts, said the latest research dealt with many of his criticisms. But he urged people to “treat this study with caution as the debate has not yet been resolved and there is clearly much work still to be done”.

Prof Pacey pointed out too that the reported decline from 99m to 47m sperm per millilitre still left the average count within what fertility clinics regard as the “normal” range.

Although the study does not examine likely reasons for the decline, “previous studies have associated low sperm count with environmental and lifestyle influences, including prenatal chemical exposure, adult pesticide exposure, smoking, stress and obesity,” said Dr Levine.

But there is little hard evidence available to disentangle possible factors. “Research efforts to identify these causes need to be redoubled and to be non-presumptive as to cause,” said Professor Richard Sharpe of Edinburgh university.

In northern Europe today more than 15 per cent of young men had a sperm count low enough to impair their fertility, Prof Sharpe added, and “this is likely to get worse rather than better”.

The combination of declining male sperm counts and a growing delay in couples trying for a baby — often until the woman is in her 30s and her own fertility is declining — created “a double whammy” for natural conception in modern western societies, he said.