Fox, who became the first openly transgender athlete in the history of MMA when she came out last year, has won 5 of her past 6 matches.
However, Women's Bantamweight Champion Ronda Rousey says she won't take on Fox, because she doesn't think there is an "undo button" on gender.
Rousey said: "In Fallon Fox's case, I think she does have an unfair advantage.
"It's a case by case scenario thing. I've tried to research it a lot.
"I feel like if you go through puberty as a man it's something that you can't really reverse. You can't just reverse that, there's no undo button on that.
