US scientists believe they have discovered why boys born too early struggle more with schooling in later life than premature baby girls.
The boys had smaller volumes of white matter
Doctors have known premature baby boys fare worse than premature baby girls and that both have smaller brains than babies born at the normal time.
Now a team at Stanford University have found specific brain areas are much smaller in preterm baby boys.
Their findings appear in the journal Pediatrics.
Compared with girls, premature boys tend to struggle more with speech and language and can have a harder time at school as a result.
Dr Allan Reiss and colleagues were interested to find out whether there might be a physical reason that could explain this difference.
They looked at the brains of 96 eight-year-old children, 65 of whom had been born prematurely.
Magnetic resonance imaging scans showed the children who had been born prematurely had smaller brains than the other children, as would be expected from previous study findings.
The volumes of both grey and white brain matter, the two forms that the brain is made of, were reduced in the premature group.
However, when the researchers divided this group's scans by gender they found a difference that has not been shown before.
The preterm girls had similar volumes of white matter to the girls born at the usual time.
But the preterm boys had much smaller volumes of white matter.
These reduced volumes were in areas of the brain that are responsible for things that reading, language, emotion and behaviour.
Dr Reiss said: "This is a remarkable finding.
"In males, the temporal lobe and the deep cerebral region of the brain are preferentially affected.
"This is very interesting because it turns out that individuals who are born preterm often have particular problems in language-based areas, and the temporal lobe is one of the seats of language.
"Researchers have hypothesised that white matter might be preferentially affected, but sex-based differences have never been clearly shown until now," he said.
Dr Reiss hopes in the future it might be possible to protect premature babies' brains against this volume difference.
"We should try to figure out a way to stimulate white matter growth in the brain of a preterm baby or develop a partially protective agent," he said.
Dr Paul Fearon from the Institute of Psychiatry, who has carried out similar brain volume studies in adults who have been born prematurely, said the findings were very interesting.
His research, also published in the latest edition of Pediatrics, found such adults had smaller brain volumes than adults who had been born at the normal time as babies. He did not find a gender difference.
But he said there were various explanations why that might be and he thought Dr Reiss' conclusions were plausible.
"It could be that by the time one reaches adolescence and early adulthood that changes have been compensated for perhaps.
"During adolescence the brain undergoes a massive transformation and a lot of the brain cells that are there in childhood tend to get pruned away.
"So although the findings of both studies seem slightly at odds there are potential explanations for it," he said.
"This study is a further important step in the direction of linking what specific parts of the brain are affected and perhaps in the future might point to ways of either preventing those brain insults or even treating them after they have happened," he said
A spokesman from BLISS said: "It's extremely positive that the study has been able to pinpoint exactly what area of the brain is affected.
"This is of particular importance for boys as it may help identify later school needs," he said.