A major review of scientific studies suggests there has been a dramatic drop in sperm counts among western men.
Researchers assessing the results of nearly 200 studies say the average amount of sperm produced by men in North America, Europe, Australia and New Zealand has fallen by 60% since the 1970s.
They also said the rate of decline is not slowing.
The analysis did not explore reasons for the decline, but researchers said falling sperm counts have previously been linked to various factors such as exposure to certain chemicals and pesticides, smoking, stress and obesity.
The lead researcher, epidemiologist Dr Hagai Levine, has said he is "very worried" because if the trend continues it may lead to the extinction of the human species.
Working with a team of researchers in the US, Brazil, Denmark, Israel and Spain, Dr Levine screened and brought together the findings of 185 sperm count studies from 1973 to 2011 and then conducted a so-called meta-regression analysis.
The results, published in the journal Human Reproduction Update, showed a 52.4% decline in sperm concentration and a 59.3% decline in total sperm count among North American, European, Australian and New Zealand men.
In contrast, no significant decline was seen in South America, Asia or Africa.
