I prefer to wait until the growth plates close and also to wait until the baby teeth remaining can be pulled. Through the years I have had five female Yorkies and one more with my mother. Four were spayed at approximately eight months, one at about ten months because she went into heat sooner, and my Katie was spayed at three years old because she was with her breeder until then and had been bred. An X-ray can be done to determine if the growth plates are closed, but in a Yorkie they usually close by eight months. Katie’s show breeder prefers that her puppies are spayed after the first heat. I realize that there is a higher chance of mammary tumors if you wait, but there are other factors to consider.
I find this article very helpful. These studies were done with larger dogs, but I believe they are worthy of consideration. All of my little ones love (loved) walking, and when in heat, I didn’t feel safe walking them, so this was one of the reasons I preferred they be spayed.
http://www.caninesports.com/uploads/...tions_2013.pdf