A biggest-of-its-kind study suggests that sperm counts are falling. By a lot. Researchers studied French men from 1989 to 2005, measuring their sperm counts regularly, and found that counts fell about one third in that 16-year period, from 74 million per milliliter to about 50 million. A steady yearly decrease of about 2 percent took it that low.

For the study, out today in the journal Human Reproduction, researchers used a database of 126 fertility clinics in France, then took that data and narrowed it down to 26,600 samples from men unlikely to have existing infertility problems. Along with a major dip in sperm count, the study also found an increase in the amount of misshapen sperm (i.e. bad swimmers) in the men, who were 35 years old on average.

That’s a big dip, obviously, but it still falls in the normal sperm-count range–the World Health Organization puts that number at anything more than 15 million per milliliter. That doesn’t sound so bad, but the researchers argue that the 2-percent-decrease is cause enough for concern, and point out that any decrease can hurt men’s chances of conceiving children.

A caveat: Studies on sperm counts have long been mired in controversy, as The Guardian writes, and the French study isn’t necessarily different just because it’s so expansive. The researchers may have sidestepped some issues but still run into others, like not accounting for the men’s socioeconomic status in the study, or not fully considering that doctors are getting better at measuring sperm count. As one scientist told the paper, “Sperm counting is difficult!”

If there is a drop in sperm count, it’s not clear what would be causing it, either. We don’t have the whole picture just yet.