See? Maybe its just the prankster in me, but of a girl told me she was pregnant and then fess'ed up an hour later that it was only a joke I'd be pissed, but hell man... she had you going, it was a funny joke.
That aside, I also one told my dad in Highschool I had gotten my girlfriend pregnant and he got really mad, heh. Luckily I did it in a public place and pulled the "I have to tell you something, please don't yell at me" line.. I told him a few minuts later that it was April Fool's, he wasn't as amused as I was, heh. He was, however, happy it was a joke.
On the other hand a friend of mine had a gf fake a pregnancy, but she went on with it for like 2 months before she claimed to miscarry, but she was psycho and her best friend (who I was dating at the time) later confessed to me that it was all a ruse because the girl was mad at the guy and wanted to piss him off. That kind of faking is not a joke and not very funny either.