Bleeding during pregnancy is rare in dogs. But, it does happen. There are several possible causes of bleeding during pregnancy in dogs. The most common cause is uterine tumors or hyperplasia (an overgrowth of cells). Other possible causes include hematomas, neoplasms, endometrial polyps, uterine rupture, or pyometra (a bacterial infection of the uterus). For example, a dog with uterine tumors may bleed after eating grass or other roughage. She may also have trouble walking due to the tumor pressing on the sciatic nerve. A hematoma appears as a swelling and bruise at the site of an injury. It may also form in the uterus, which can lead to internal bleeding. Uterine ruptures occur rarely and are usually caused by a large fetus pressing against the wall of the uterus. Pyometra is a bacterial infection of the uterus that can lead to severe pain and fever. This condition occurs more often in older females. In addition, there are certain breeds of dogs that are prone to developing these conditions. For example, Boxers, Great Danes, and Labrador Retrievers are known to develop uterine tumors. Bulldogs, German Shepherd Dogs, and Irish Wolfhounds are known to develop hematomas. Golden Retrievers, Rottweilers, and Doberman Pinschers are known to develop pyometra.