Unilateral cryptorchidism refers to the normal descent of a singular testis. Bilateral cryptorchidism results in the retention of both testes. Due to the thermal suppression of spermatogenesis, bilateral cryptorchids are sterile while unilateral cryptorchids are usually fertile. Saint Bernards are noted as a breed at risk. An additional phenomenon, monorchidism, may occur in very few dogs.
Monorchidism is a rare condition in which only one testis actually develops in the dog's body.
Confirmation of monorchidism may only be determined via plasma testosterone analysis. The concentration of plasma testosterone is significantly lower in dogs who lack one or both testes. Unlike unilateral cryptorchidism, the testis is not ectopic but rather, does not exist in any location. The dam and sire of the cryptorchid pup may appear to be normal while being carriers for the defect. Given the polygenic nature of cryptorchidism, it is difficult to calculate quantitative results with respect to frequency and distribution.