Many men who were born with undescended testes have reduced fertility, even after orchiopexy in infancy. At least one contributing mechanism for reduced spermatogenesis in cryptorchid testes is temperature. The temperature of testes in the scrotum is at least a couple of degrees cooler than in the abdomen, which is necessary for healthy spermiogenesis. Increased temperature has a well-documented detrimental effect on the structure and function of the testes, eventually leading to a reduced count of sperm (oligozoospermia) or even azoospermia. However, the mechanisms of reduced fertility in cryptorchid males have been found to be much more complex.
Orchiopexy performed early in infancy significantly improves the fertility potential in cryptorchid men, woever, there is still some reduction in fertility compared to healthy men. The reduction with unilateral cryptorchidism is subtle, with a reported infertility rate of about 10%, compared with about 6% reported by the same study for the general population of adult men. The fertility reduction after orchiopexy for bilateral cryptorchidism is more marked, about 38%, or 6 times that of the general population. The basis for the universal recommendation for early surgery is research showing degeneration of spermatogenic tissue and reduced spermatogonia counts after the second year of life in undescended testes. The degree to which this is prevented or improved by early orchiopexy is still uncertain.