How soon after a cat gives birth can you get them fixed?

United States
@freedomg (1687)
August 17, 2008 11:04am CST
I have two wonderful kitties with one problem, they keep getting pregnant. Every time I go to take them in to get fixed they are prego again! I tried keeping them inside but they have figured out that my Dad can't block them at the door and they wait for him to go outside to smoke a cigarette. Next thing I know they are knocked up. I know there are times that it is dangerous to get them fixed and I want to protect them. The vet will only answer questions during an office visit(yeah he wants the $). So any info you wonderful people might have will be greatly appreciated.