Source: The New York Times - Link
But the idea that there is a sudden influx of transgender competitors who are dominating women’s and girls’ sports does not reflect reality — in high school, college or professionally. Sports associations like the N.C.A.A., which has promoted the inclusion of transgender athletes, have policies in place to address concerns about physical differences in male and female biology. The N.C.A.A., for example, requires athletes who are transitioning to female to be on testosterone suppression treatment for a year before they can compete on a women’s team.
There is no evidence for inequity at the high school level. Girls have been competing in “boys” sports for years. A true trans girl would likely have no advantage at all.