An undescended testicle (cryptorchidism) is a testicle that hasn't moved into its proper position in the bag of skin hanging below the penis (scrotum) before birth. Usually just one testicle is affected, but about 10 percent of the time both testicles are undescended. An undescended testicle is generally rare in full-term babies, but common in baby boys born prematurely. Not seeing or feeling a testicle where it's expected to be in the scrotum is the main sign of an undescended testicle.