Is it possible for a female swordtail fish to change its sex? I had what looked like a pregnant female swordtail in my tank for the past month. When looking in my tank today, I noticed a very distinct sword.
A female can grow a sword. Particularly as it gets old. It's still female though.
More common, I believe, are "stealth" males that don't develop a sword for a long time. I've read of cases that took well over a year.