Kittens can’t hear for 12 days after they are born.
Also, when the kittens are born, the mother cat eats the placenta. It contains some nutrients that are good for the cat (or something like that).
Source: National Geographic Channel show about Cats in the Womb

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I believe that is one of the many uses of the cat’s purr. Kittens can’t hear, but they can feel the mothers purr to tell where she is.