How do you know when your cat is pregnant?

You need to be on level 15, and tap the "BREED" button.

Hit the breed button

She doesn't get fat or anything though...

Tap the Breed button right above the open button when you’re level 15. You have to have another cat in your family that is not your gender, but you’re cat doesn’t look any different than normal. And just so you know it starts a timer for 15 minutes and when it finishes there’s a button that says “birth” once you press that a kitten appears next to your mate