The first sign of heat you will notice is blood droplets on the floor or wherever she has been sitting. Her
vulva will be swollen to a much larger than normal size. Normally, the female will still not allow a male to breed her during the phase when she is bleeding. It's after that that she becomes fertile and willing to breed. All dogs are individuals, however, so as long as her vulva is swollen, she should be kept away from all males to prevent her from being bred.