What Is A Dog In Heat?

If you are asking about the heat cycle of dog then there are four stages of canine heat cycle. First heat cycle can start after 6-8 months of age. And dogs usually come in to heat twice in a year.
First stage of canine heat cycle is called proestrus. At this time, vaginal bleeding starts and swelling of vulva takes place. Second stage is called estrus during which she is ready and mating takes place. Ideal time for mating is on 11-15 day starting from the first day of bleeding or when your dog starts to accept males and show flagging behavior.
Third stage is called diestrus which can last for 56-58 days if she becomes pregnant and in case of no pregnancy, this stage may last for 60-100 days. Fourth and last stage of canine heat cycle is anestrus which is a period of no attraction for and from male dogs. This period can last for 4-5 months.