You're tempted to adopt one of those adorable 4-week-old kitties right on the spot, but wait. Separating that sweet furball from mamma and littermates too soon can harm her health and future social skills. Kittens shouldn't be taken from mamma's care until they're at least 8 to 10 weeks old.
Separating nursing kittens from their mother can be life-threatening. Their ability to eliminate wastes depends on mamma at this stage. Her milk not only is vital for their nutritional needs, but it provides antibodies to protect them against disease during the first weeks of life, when they have no defenses of their own. Kittens this young can't regulate their body temperatures, so they depend on the warmth of their mother and their littermates. Many cat breeders refuse to sell kittens until they're 12 to 16 weeks old, to ensure their immune systems are fully developed.
At around 4 weeks old, kittens start the transition from mother's milk to solid food, which is the beginning of a new independence. The weaning process is usually completed by the time they're 8 to 10 weeks old. Kittens should remain with their mother so she can handle the weaning process for the most effective transition. Kitties learn to feed themselves by watching their mother eat and trying to copy what she does. During the weaning stage, kittens should be gradually introduced to canned and dry kitten foods. Food should be kept in an area away from their mother so they become accustomed to spending more time away from her.
Kittens learn social skills by mirroring those of mamma cat and by interactions with littermates. By living with and copying their mother, they learn how to clean and groom themselves and how to use the litter box. They learn predatory skills. Play-fighting with mamma and siblings, they develop their physical coordination and strength as they begin to explore the world outside the den. Separated from their mothers too early, kittens lose the opportunity to develop these skills.
By the time you do adopt that little ball of fluff, she and her littermates should have visited a veterinarian for a health checkup. A vet should check and treat kittens for roundworms and other parasites when they're 2 to 3 weeks old, with a second treatment at 5 to 6 weeks. At 6 to 8 weeks, kittens should begin a vaccination series that is completed at around 16 weeks, according to the American Society for the Prevention of Cruelty to Animals.