A female-looking dog may be discovered to have vestigial testes upon ultrasound! These animals are nearly always infertile, even if the testes are removed. Though rare, intersex conditions have been noted in many species, such as horses, cows, and even human beings.

A female dog may also have physical abnormalities which prevent her from getting pregnant. For example, she can have vaginal anomalies, such as tumours, inflammation, or a persistent hymen. She can also get endometritis, where the innermost layer of her uterus becomes infected with bacteria. Since only one layer is affected, the dog will usually not show clinical signs. However, she can go on to develop metritis, where all three layers of her uterus are now heavily infected with bacteria. This is a systemic disease, affecting her entire body, since the bacteria can now escape into her bloodstream.

Thus, she will show signs such as fever, depression, vomiting and diarrhea. Her uterus may also may be inflamed, irritated or filled with pus, a condition known as pyometra. Pyometra occurs when there is a bacterial infection while a bitch is in diestrus. Diestrus is the period of a female’s estrus cycle when the hormone progesterone is highest. Estrogen is thought to increase the capability of the uterus to fight infection; thus, in diestrus, when estrogens are low, the uterus is at a high risk of bacterial invasion. Pyometra is a veterinary emergency; a dog suspected of having it needs to be seen by a veterinarian immediately.