Over the past 40 years, sperm counts in men from North America, Europe, Australia, and New Zealand have dropped by over 50 percent, researchers announced on Tuesday.

Researchers examined 7,500 studies conducted between 1973 and 2001. They selected 185 studies with consistent counting methods and performed analyses on data from almost 43,000 men.

Their findings, released in Human Reproduction Update , showed that sperm concentration fell from 99 million per milliliter to 47.1 million per milliliter during the span of the study, indicating a 52.4% decline in sperm concentration and a 59.3% decline in total sperm count for men in these nations. What’s more, the scientists found that the decrease in sperm count is not slowing. As sperm counts have implications for morbidity and mortality, these results raise potential concerns for health and fertility.

While previous studies on sperm count were criticized for not accounting for confounding variables like age, sexual activity, or infertility, this study used consistent sperm count methods and broad data sets from many locations to address these concerns.