Sperm counts of men in developed nations have fallen by 52 percent in the last 40 years.
An analysis, which combined the results of 185 studies carried out between 1973 and 2011, also showed that the average sperm concentration had dropped by nearly 60 percent. The reasons for this decline remain unclear.
'This study is an urgent wake-up call for researchers and health authorities around the world,' said Dr Hagai Levine, the study’s lead author and researcher at the Hadassah Braun School of Public Health and Community Medicine, part of the Hebrew University of Jerusalem.
The research, which was published in the journal Human Reproduction Update, analysed studies encompassing over 42,000 men. After taking account of other influencing factors such as age and time since last ejaculation, the team found that in men from Western countries including Europe, North America and Oceania, average sperm concentration fell from 99 million per ml in 1973 to 47.1 million per ml in 2011, which is below the threshold for fertility. While the study did not examine sperm quality, Dr Levine called this 'particularly concerning'.
The decline in sperm count was not seen for men in other countries, leading some to speculate that environmental factors might be responsible. However the researchers did not look at the causes of the decline, and they also caution that far fewer sperm count studies have been carried out in developing countries.
Male fertility experts have welcomed the study for addressing many concerns regarding previous studies of sperm quality, such as only including studies that counted sperm using modern equipment, and excluding studies that recruited men from fertility clinics. Professor Richard Sharpe of the University of Edinburgh said the study 'is about as close as we are going to get' to being sure that sperm counts are declining.
The research also showed that the rate of decline in sperm counts has continued beyond 1995, indicating that the drop is ongoing. 'The continuous nature of the decline is of as much concern as the decline itself, given that we still do not know what lifestyle, dietary or chemical exposures might have caused this decrease,' added Professor Sharpe.
Experts agreed that more research is needed in this area to identify the underlying cause of the decline. Professor Daniel Brison, scientific director of the Department of Reproductive Medicine, University of Manchester, said 'this has major implications not just for fertility but for male health and wider public health. This study should act as a wake-up call to prompt active research in this area.'