In most children with this health issue, it's not known why the testicles fail to drop. It may be because the testicles aren't normal to start with. In other cases, there's a mechanical problem. The testicles drop but miss the scrotum, ending up next to the scrotum instead. These are called "ectopic testicles." Or it may be that the baby's hormones can't stimulate the testicles normally. No studies have shown that the problem is caused by something the mother did or ate during pregnancy.
Sometimes the testicles drop but don't attach in the scrotum. Then, when the boy grows, it becomes clear that the testicles aren't attached. About 1 of every 5 cases of undescended testicles are found once the boy is no longer a baby. For this reason, all boys should have the location of their testicles checked during each annual physical exam.