I'm planning to tell my wife on april fools that her best friend was pregnant with my baby
Why would you tell her that.
It's april fools day, I feel like fucking with them
I get your point, but it's still mean to play such pranks on people. You should come up with something less hurtful.
I don't care. I'm doing it.
You should care more about your wife's feelings. Telling her this will upset her even if it is a joke. She might even become suspicious of you afterward.