A dog’s oestrous cycle is known as a heat cycle

A dog’s oestrous cycle is known as a heat cycle. Once a dog reaches maturity, between 9 and 12 months, they will come into heat twice a year. A dog’s heat cycle is made up of four stages, proestrus, estrus, diestrus and anestrus.
The first stage is proestrous and lasts roughly nine days. In this stage, the dog’s vulva will swell and will secrete a red/brown discharge. Proestrous is the preparation of the body for pregnancy. In this stage, the female will start attracting males although will not be interested in mating. She will turn away males and possibly snap at males who try to mate. The females body will start producing oestrogen 30 days before this stage and stop producing it toward the end of proestrous. As oestrogen stops, progesterone begins to rise.
The second stage is estrus and also lasts nine days. In this stage, the vulva is still swollen however the discharge is now a light pink colour. Progesterone levels start rising in late proestrous and continue to rise in eetrus. LH also rise. This rises quickly when oestrogen stops being produced. LH stimulates the release of eggs. Now that the female is ovulating, she will be very interested in finding a male. Her behaviour will become very playful and “flirtatious”. Most will play bow and turn their rear end to the male with their tail to one side. This movement is called flagging. in this phase, copulation will occur.