During the unborn male's development, the testicles are located in the belly.  They slowly move down into the scrotum during the seventh month of pregnancy. When they drop, they pass through a small passage that runs along the belly near the groin called the inguinal canal. Once through the inguinal canal, the testicles reside in the scrotal sac. Since the scrotal sac is cooler than the body temperature, it is the ideal location for the testicles because they function better at this cooler temperature. 

Undescended testicles (cryptorchidism) refer to a condition in which the testicle has not dropped and cannot be brought into the scrotum with external manipulation. This occurs in 3 percent of newborn males and up to 21 percent in premature male newborns. Most testes drop by 3-4 months of age. 

The undescended testis can be found in the belly, the inguinal canal or other places, but most are found in the inguinal canal (80 percent). About 10-15 percent of all cases are bilateral (involve both testicles). 

This condition can run in the family. About 14 percent of boys with this condition come from families in which another male has it also. Six percent of fathers of males with undescended testis have also had this problem. 

It is very important to explain that "retractile testicles" are not undescended testicles. If a testicle can be brought down into the scrotal sac, even if it bounces back up again upon release, it is a retractile testis. These retractile testicles are in the scrotum at other times and do not need treatment.