One-third of children given cochlear implants had actually passed the newborn hearing screening test at the hospital before being later diagnosed with hearing loss, a new study finds.

The study raises worries that some parents may be falsely reassured that their babies don't have hearing problems when in fact they do, thereby delaying getting the children the help they need.

Researchers at Children's Memorial Hospital in Chicago looked at the records of 127 children with severe hearing loss and who were being given cochlear implants.

The implants are surgically implanted electronic devices that can restore some hearing. With the implant, sound no longer travels into the ear canal but is instead picked up by a microphone above the hear and sent through the device's processor deep inside the ear's hearing centre.

The researchers found that one-third had passed the newborn hearing test, only to be diagnosed later in infancy or early childhood. The results are published in Archives of Otolaryngology – Head and Neck Surgery.

The researchers say the newborn screening test may not be at fault. Some hearing loss can simply develop slowly and this type of hearing loss cannot be detected immediately after birth.

Some forms of hearing loss with a delayed onset are due to problems in the inner ear or central processing centres of the brain. Babies at risk for progressive deafness include those who had congenital cytomegalovirus infections, severe jaundice, and those who were on prolonged assisted ventilation in a neonatal intensive care unit.

The researchers say their findings are important, because parents and pediatricians often don't consider hearing loss if the child passed the newborn screening test. But the earlier that children with hearing problems get help, the better they'll be at developing language skills.

The children in this study who had failed the hearing screenings were diagnosed at an average age of 5.9 months.

The authors note that before newborn hearing screening programs, children with profound deafness usually weren't identified until 17 to 24 months of age.

"The situation raises the issue of whether repeating mandatory hearing screening for all children before 1 year of age would be beneficial," the study authors write.