From everything that I have personally read, they say it depends on the dog (size, weight, etc., etc., etc.). Destiny, our Dalmation, did get her first heat in before we were able to get her fixed. We first noticed her spotting one week before her 1 year birthday. So, this is not a technical answer, but simply what our baby did. I'm sure others on here will be able to help you more. I would tell you what out puppy books say, but they're at home and I'm at work right now. As I said though, I'm sure others will be able to help you more!