All rattlesnakes are viviparous, meaning they bear live young, rather than oviparous (egg laying). So, the first part is true. The second most assuredly is not. No animal gives birth through its mouth. All snakes give birth or lay eggs through their cloaca, an opening at the base of the tail, which is also where they excrete waste products. The same is true of fish, amphibians, and birds. If a pregnant rattler dies and the young are nearly ready to be born, they can be removed surgically with success, but a dead animal cannot give birth on its own.