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While spaying is traditionally recommended for kitties between 4 and 6 months of age, spaying as young as 8 weeks old is considered safe, according to Cat Channel. Many animal shelters actually spay kitties prior to adopting them out at this age.
The mother cat can be spayed as soon as the kittens are weaned, usually when they around 10 to 12 weeks old. Some shelters will have their kittens neutered at 8 weeks - 12 weeks or 2.0 lbs also. Neutering does not make cats fat or change their personalities regardless of when it's done.