I fight with my vet over this issue a lot. I like to wait until after the first heat cycle. She says it is much easier on them and her to do them before, also says it cuts the risk of some cancer. Having a rescue done before 9 weeks with a few issues I believe caused by it, I will wait. It is not advisable to spay if you think they are getting ready to come in heat. I hear because of increased blood supply could make if difficult.
As I sit here with my chihuahua on 2nd heat cycle, I assure you she will be spayed before the next one. I shut my vet up by asking if she would like to give her very young daughter a hysterectomy? I'm older and realize how important hormones are, she shut up pretty fast.