The sperm count of men in Western countries has been declining precipitously with no signs of “leveling off,” according to new research, bolstering a school of thought that male health in the modern world is at risk, possibly threatening fertility.

By examining thousands of studies and conducting a meta-analysis of 185 — the most comprehensive effort to date — an international team of researchers ultimately looked at semen samples from 42,935 men from 50 countries from 1973 to 2011.