There was a story I read a while back about a guy that gets a magic Halloween costume that makes him look like a woman, he decides to up the ante and be a pregnant woman (the catch was he has to take it off by a certain time or he will turn into the costume…he doesn’t take it off in time. Anyone know the story I mean and where I can find it.

I think that's The Costume by Levia Neo: https://overflowingbra.com/storyratings.htm?rateID=2322