They know :) animals just know, but I haven't heard of them sensing labor.. My dog is the opposite, sadly. He kinda looks at me funny and won't come near me :( lol and sometimes he tries to lay with me by my tummy and scoots back to the point I have to move him because he squishes me haha. But then again, he knows when we're getting ready to leave somewhere and he'll go lay by himself all alone and basically pout because he doesn't want us to leave.
Dogs and cats can hear and feel the babys heartbeat, and i bet when the baby comes your pets will be really protective
I dont think it could be labor coz my dog started being like that early and I'm only 21weeks now and she's getting worse but I think its the baby they are attracted to and being protective already