The testicles (testes) actually form in the abdomen during male fetal development. They normally move down into the scrotum in the final months before birth. But in some male fetuses, one testicle — or sometimes both — fails to fully descend. At birth, it remains up in the abdomen or only part way down toward the scrotum. Doctors refer to this as an undescended testicle (cryptorchidism). Up to 30 percent of boys born prematurely have an undescended testicle.

This condition usually corrects itself without treatment within the first two years of life. In some cases, injections of a hormone called human chorionic gonadotropin may help. But if the testicle doesn't seem likely to descend, treatment is surgery to relocate it to the scrotum. In a newborn male with two undescended testicles, a doctor may perform medical tests to confirm the baby's sex and to identify the location of the testes.

A male with only one healthy testicle can still have normal erections and make adequate sperm to conceive a child. Adult males with a testicle that never descended are at increased risk of testicular cancer. The risk remains even if the testicle has been surgically relocated. A testicle that stays in the abdomen could twist and impair blood flow to the testicle (testicular torsion).