A lot of breeders who sell active kittens have a clause in the contract stating..
breeding girl..
if she goes to stud 3 times and fails to get pregnant then she is rendered infirtile so a replacement kitten of the same quality is offered.
same for stud boys..
not many boys are offered for stud as most have a plop breeding life..
but if they are proven infirtile then a replacement is often offered.
hope this helps x