An unspayed female dog will go into heat about once every six months. During this period, she is looking for mate, and her behaviour and body will change. Recognising when a female dog goes into heat is an important part of breeding dogs. If you do not want puppies, be aware of the signs of a dog going to heat can, which tell you when to keep your dog close to home.
The vulva is the dog's genital opening. When a female dog goes into heat, her vulva will be swollen and she will frequently lick it. Remember, though, that a swollen vulva can also be a sign of allergic reaction or infection.
When a dog begins her heat, she will bleed from her vagina. The flow of blood can be heavy during the first few days, but it will usually slow over the course of a week. At the beginning of the week, the blood is dark red, but by the end, it will usually be more diluted and pink in colour.
A female dog who is in heat will urinate frequently. In some cases, she will do this in the house even when she has been housebroken. The increased urination is meant to spread her scent around and to let male dogs know she is in heat.
When a female dog becomes receptive to mating, she will hold her tail off-centre, to the left or the right, in a behaviour known as "flagging."
A dog who is in heat will likely have swollen nipples because of the change in hormones in her body. She might also produce a small amount of milk.