Testicles are usually formed in the abdomen when a baby is in the womb. In the last few months of growth, the testicles slowly descend into the scrotum through the inguinal canal, which resembles a passage. If the baby has an undescended testicle, it means that the process is somehow interfered with. Why this happens is not known. It is thought that a mix of environmental factors, genetics and maternal health may be held responsible.