As Phil just stated, it's best to wait for the growth plates to close. If a female goes into her first heat, you need to wait for approximately two months before she can be spayed. I also preferred to wait because I had retained baby teeth pulled at the same time as a spay, and it meant less teeth needed to be pulled.
Here is a link to an informative pdf. There are many other vets who support his findings.
http://www.caninesports.com/uploads/...tions_2013.pdf