Passing a newborn hearing test is no guarantee against deafness.
U.S. researchers found nearly a third of kids with cochlear implants - a device that transmits sound directly to the auditory nerve - had initially checked out on mandatory screening.
"This is important information to remind everyone that just because a child passed their hearing screening doesn't mean that you're out of the woods," Dr. Nancy Young, the lead author of the study from Children's Memorial Hospital in Chicago, told Reuters Health.
screening tests were put in place starting in the 1990's with the hope that deaf kids would get hearing devices sooner if they were diagnosed earlier.
The U.S. Preventive Services Tasks Force, a federal expert panel, also recommends hearing screening of all newborns, which previous research suggests improves child development.
But the testing programs might not be as successful as hoped because they miss babies who have some hearing, but go on to develop deafness over the next couple years.
"The problem is, this is very difficult to identify," Young said. "By the time it comes to the paediatrician's attention, by the time the family is concerned, there already might be a significant delay."
Newborn hearing screening is done by a nurse or another hospital staff member soon after birth. The test measures a baby's ability to hear sounds at a few different frequencies.
To evaluate the effectiveness of mandatory screening in their state, Young and her colleagues reviewed the medical charts of almost 400 Illinois kids. All of them had been diagnosed as deaf and received cochlear implants at Children's Memorial Hospital between 1991 and 2008.
About two-thirds of the babies in the study were born before hearing screening was mandated law in Illinois in 2003; the rest were born later.
After the law went into effect, the number of babies who were screened jumped from 33% to 85%.
Passing screening meant that it took longer for kids with hearing problems to be diagnosed and to get the appropriate treatment. When babies failed their hearing test, they were diagnosed with hearing loss at an average of 6 months, compared to more than 18 months for those who passed.
"There's always going be some that pass their screen and develop hearing loss," Dr. Jeffrey Carron, who studies hearing in children at the University of Mississippi Medical Center in Jackson, told Reuters Health.
However, Carron, who was not involved in the current study, said he was surprised by how high that number was.
Close to 30,000 deaf children in the U.S. have received cochlear implants, which are surgically attached under the skin behind the ear. The device collects outside sounds using a microphone and translates them into electric impulses, which are then fed directly to the auditory nerve that carries the signals to the brain.
The first few years of life are a critical period for development of the brain, and sound plays a vital role in that development, Carron told Reuters Health.
If you miss the earliest chance to diagnose hearing loss
, "you're going to play a lot of catch-up, you're going to have to have a lot more therapy (and) the child is going to need a lot more special assistance," he said.
It's not clear where to go from here to fill the gaps in universal infant screening.
It's likely that a 2nd round of screening, for example at one year, would pick up more kids with hearing loss - but much more research is needed to see if it would be worth the extra cost, Young said.
The message right now, added Carron, is that doctors and parents should talk about what to expect as babies are starting to speak and communicate.
Parents who are worried their babies might be developing too slowly should take them in for evaluation right away, he said.
Was your baby's hearing tested at birth?