At some point after a female dog comes into heat, there is a bloody vaginal discharge. The amount of bleeding differs from dog to dog. In some cases it is not even noticeable to the dog owner.
A female dog's estrus cycle is the period during which she is able to become pregnant. These generally occur every six months, although there is some variation among breeds and individual dogs. Smaller dogs generally have less time between cycles, while big dogs have more. During each cycle, a dog is in heat for two to three weeks. In addition to discharge, another physical sign of estrus is swelling of the vulva, although this, too, is not always apparent to owners.