The most common cause of hearing loss in newborns is cytomegalovirus, an infection that may not cause any symptoms in a pregnant woman but can affect her developing fetus, especially if she contracts it in the first trimester. Hearing loss from cytomegalovirus can be severe, but may develop only after the child is born. Other causes: oxygen deprivation and medications. Before immunization, rubella was the major menace. In cases that do not involve a syndrome, more than 100 different genes may be implicated.