This research suggests that there are many more children with hearing problems than was thought

Dr Heather Fortnum, Institute of Hearing Research

The current Health Visitor Distraction Test is not carried out until a baby is eight months old.

And as well as failing to pick up children with hearing impairment, the Royal National Institute for Deaf People says it fails to detect almost half of the 840 babies born deaf each year.

Undetected hearing loss can affect communication skill, educational ability and the child's quality of life.

The researchers say parents and teachers should be aware hearing problems could affect children - even if they have passed earlier hearing tests.

Extent of the problem

The team from the Medical Research Council Institute of Hearing Research, in Nottingham, looked at data on 17,000 children who had permanent hearing impairment.

The information came from health professionals and teachers dealing with children with hearing problems.

The researchers used that data to estimate the potential numbers of children with permanent hearing impairments across the UK.

Existing predictions say just an additional 16% children with problems are not picked up by existing tests.

But the Nottingham team say their findings suggest the figure could be as much as 50-90%.

Some children's problems will develop after the neonatal tests. They might have developed their impairment late, or it may be a gradual worsening of hearing that was not severe enough to be picked up at the early tests.

Others may have an inherited hearing problem which was missed, or develop an impairment after suffering a condition such as meningitis.

Children who cannot hear sounds over 40 decibels are classed as having significant hearing impairment.

Screening

Dr Heather Fortnum, who led the research, told BBC News Online: "This research suggests that there are many more children with hearing problems than was thought."

But she said, even with universal neonatal testing, some children would slip through the net, and said it was crucial parents and teachers were not complacent.

In an editorial in the British Medical Journal, Adrian Davis, director of the NHS Newborn hearing programme, who was also involved in the research, said: "The benefits of universal new-born hearing screening for children with permanent hearing impairment are that early identification is associated with better expressive and receptive language, speech and social and emotional development."

But he adds there is a need for better screening and identification of older children who are experiencing problems.

James Strachan, chief executive of the Royal National Institute for the Deaf (RNID) said the findings were proof it was vital to test all new-born babies for deafness, improve school-age screening and have easier access to testing when parents are concerned about their child's hearing.

He said: "There is now no excuse why any baby born today should not be tested for deafness at birth.

"Undetected deafness in children has a serious and long term impact on a child's development of communication, language and speech skills and can be very damaging to the child's life chances."