I have a feeling they will not do a cat in season, so you may have to wait a week or so until she is definitely not. But even if she is in kitten, they willspey her. One of my farm cats had kittens, we weaned them and booked her in. This was probably 12 weeks after the kittens were born. Vets office rang to ask if we wanted the op to go ahead, as she was in kitten again. I said yes, take them away, she cannot be very far gone surely .....so they did. Had the birth been iminent, my answer would have been - no wait.