Dogs will experience their first heat cycle between 6-12 months of age, though most come into heat between 6-8 months of age. It is important to spay your dog at 6 months of age to prevent heat cycles. Spaying a dog before the first heat cycle reduces the chance of mammary cancer to almost zero. This also prevents unwanted litters--it is amazing what a male dog will do to get to a female in heat.
