I don't get why this is called rare in the article. It's not called that in the actual study, so the article seems to do some editorializing: https://www.acpjournals.org/doi/10.7326/M21-4389 Looks like of the kids taken from dads taking metformin, .9% had a genital defect, which seems pretty high to me (defect in control is 0.24%). Granted, not as high as Thalidomide, but still, 9 in 1000 are not great odds.
I think it means the genital defect is normally rare, but is much more common in these kids?
From NIH.gov:
"In the United States, a rare disease is defined as a condition that affects fewer than 200,000 people in the US."