If the bitch fails to conceive after successful matings, the most probable cause is a uterine infection – either pyometra or cystic endometrial hyperplasia. Other possible causes are brucellosis or canine herpesvirus. These two sexually transmitted diseases cause infertility by preventing conception, or by causing early loss of pregnancy or spontaneous abortion.
Infertility may result from vaginal strictures and neoplasms that prevent intromission or the ability to tie. A blockage in the oviducts can prevent the union of egg and sperm. Tumors and congenital anomalies of the uterus can interfere with conception and embryo implantation. Lethal genes and chromosomes produce defective embryos and malformed fetuses, which are reabsorbed or expelled.