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 uncommon in general, but common among baby boys born prematurely.
The vast majority of the time, the undescended testicle moves into the proper position on its own, within the first few months of life. If your son has an undescended testicle that doesn't correct itself, surgery can relocate the testicle into the scrotum.
Undescended testicle care at Mayo Clinic