Female dogs of all breeds typically stay in heat between five and 13 days. During this time, a female dog is willing to mate and can become pregnant.
Dogs usually go into heat two or three times a year. The first signs are seen four to 20 days before heat actually begins. During the proestrus period, the female may bleed slightly from the vulva or have a vaginal discharge but will not be interested in mating. When she goes into heat, her vulva will be swollen and she will show receptiveness by backing toward a male. If she does not get pregnant before her heat passes, she will enter anestrus and will not come into heat again for 60 to 90 days.