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The usual sign that she is finished is that she will get out of the box to get a drink of water and walk around for just a minute or 2. She may not eat anything, but she will be ready for a drink. She will also start really giving the kittens a thorough cleaning. Finally, if it's been more than a couple of hours since the last kitten was born, and you don't see any signs of her pushing, then she's undoubtedly finished.
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