Heat, which is also known as the Estrus period, is a reproductive stage that unspayed female dogs go through. During this stage her eggs mature, making her fertile and able to produce puppies. Your dog will have different behavioral and physical changes during this stage, due to hormonal changes in her body. These changes will allow you to figure out whether she is in heat or not. 1. Understand the basic reproductive cycle of your dog. When wondering if your dog is in heat, it first helps to understand her normal hormonal cycle. When a female dog reaches maturity, and is physically ready to reproduce, her estrogen levels increase, initiating the release of eggs from her ovaries. If mating occurs during this period, the eggs can then be fertilized and an embryo can be produced. [1] Female dogs, will generally go into heat during puberty, at around 6 to 24 months of age depending on breed. Smaller breeds generally come into heat earlier than larger breeds. ...