Q: Should I let my dog have a heat before I spay her?
A: Medically, it’s better to spay your dog before her first heat. It greatly reduces the risk of mammary tumors. People who wait to spay their dogs until after their second heat greatly increase the risk of mammary tumors in their pets. Once they’ve had several heats, intact female dogs have a one out of four chance of developing mammary tumors.
http://pets.webmd.com/dogs/guide/spaying-or-neutering-your-dog-faq
