The heat cycle of the female dog has three parts and lasts about 3 weeks. The first part lasts anywhere from 3-14 days during which a bloody discharge and swelling of the vaginal area are usually seen. Next the dog will become receptive to being bred and actively may seek out a male dog. This is the time when she can become pregnant. The discharge may change color at this time and become lighter. Finally, the discharge will become clearer, the vaginal swelling will diminish and the dog will no longer allow herself to be bred. While your dog is in heat you must watch her carefully to avoid unwanted breeding and subsequent puppies.