I always heard it as "a cat will steal the baby's breath", which is a bit more supernatural than mere physical smothering. That might have originated in seeing

I always heard it as "a cat will steal the baby's breath", which is a bit more supernatural than mere physical smothering. That might have originated in seeing a cat sniffing a baby's exhaled breath. Much like a cat will sometimes sniff around your own mouth or nose, if its face is near yours.