My vet wanted to wait until Bella was a year old because she was so small. She ended up losing her last tooth during the preop lab draws. They were holding her down and i looked on the table and said...oh look a tooth
They saved me 20 bucks!
She ended up having her first heat before I had her spayed. I was a little disappointed because I read that increased her risk for breast cancer. But I know there are tons of happy healthy dogs that are spayed later in life.
I dont know the science behind it all... well I guess i was a lot of help