Children with delayed speech tend appear to use a different part of their brain to listen, research suggests.
One-year-olds should begin to talk
Miami Children's Hospital used sophisticated MRI scans to compare the brains of children with and without speech problems.
More activity was found on the right side of the brains of children with delayed speech. The others tended to use the left side.
The research is published in the journal Radiology.
Lead researcher Dr Nolan Altman told BBC News Online that language skills were associated with the left side of the brain.
He said: "This finding of right sided activation in speech delayed children is significant as it may indicate a different process of maturation or laying down of language pathways in these children's brains."
Scans were taken as the children, whose average age was four-and-a-half, listened to audiotapes of their mothers.
The researchers also found that the children with delayed speech had less total brain activity.
This, they suggest, indicates that they are less stimulated by language than normal.
Children typically say their first words by their first birthday, and advance to simple sentences between 30 and 36 months.
The researchers say their findings suggest that children with speech problems should be given help as soon as possible.
They suggest that if a one-year-old child has not made verbal sounds - or if his or her speech is extremely unclear compared with that of children of similar age - then it may be advisable for parents to seek advice.
Dr Altman said: "We are trying to find ways to diagnose speech and language delay at an early age definitively so treatment and follow up can be earlier and quantified.
"This is a daunting process since so little is known about the normal process of language development and laying down of processing of the brain.
"But with MRI, radiologists may be able to help diagnose, guide and monitor treatment of children with these complex disorders."
It is estimated that 2% of children have a condition that may case speech delay.
These can include emotional or behavioural disabilities, birth complications, cleft lip or palate, developmental disabilities, hearing loss or lack of environmental stimulation.
Dr Altman said the next step was to expand the study and develop a reliable test to diagnose language delay.
"A valid test identifying language delay would be valuable to both the practitioner and the child.
"Alternatively, after the child goes through speech therapy or another intervention, we can re-scan to see if the brain appears more normal."
Sue Roulstone, of the Royal College of Speech and Language Therapists, told BBC News Online that in the UK most children with speech problems were referred on by health visitors, who monitor their language development in the early years.
She said that for speech therapy to have any effect it often had to be well-targeted and intensive.
"There is a lot of developmental noise going on in the early years and a lot of children who appear to have problems get better on their own," she said.
"We are still working hard to try to identify those who need help, and those who will improve by themselves."
Ms Roulstone said the left side of the brain was thought to be closely linked to processing of language.
However, she said the fact that children with speech problems appeared to be using the right side of their brains to "listen" was not necessarily a bad thing.
It may indicate that their brains had taken steps to overcome difficulties with the more usual language processing centres.
"They may be engaging some other part of the brain to assist with the process of understanding language," she said.