Can A Cat Get Spayed Right After Delivery? by: Anonymous
It is possible to spay a cat that's just given birth. It's often done to female dogs that have a C-section (usually as an emergency) when the owner doesn't intend to breed her anymore. It actually makes the c-section easier because you don't have to worry about keeping the uterus in tact or about uterine infections post op. You can focus on mom's and maybe pups' well-being instead. The dogs tend to recover well from it considering the C-section plus spay was usually done to save their life.
However that doesn't mean it'll work as well on a cat. Cats usually don't need c-sections in the first place. And of course there will be more complications if sugery is done now.
I'd say it's bst to wait until the kittens are weaned and then spay mom. Keep her indoors in case she emters heat again in the meanwhile.