You dreamed it. Our brains come up with loads of bizarre things while we sleep.

Tell her about the dream if it will make you feel better though. There's no harm in sharing your dreams. You could even tell her that you woke up and were half-convinced it was real!

But don't ask her to take a pregnancy test, nor ask her if she's pregnant. You've got no basis to think that outside of your dream!

(Hell, if dreams always relayed truths, I would have no memory of myself as I've had my memory wiped multiple times in nightmares! Yes, I jest... but I've had loads of very realistic nightmares. When I wake up, I remember they're nightmares. I may or may not tell the person involved about what a nutty nightmare I'd had.)