Babies born prematurely show the effects years later, with parts of their brains significantly smaller than normal when they are 8 years old, U.S. researchers said on Monday.

And boys are more affected than girls, the team at the Stanford, Yale and Brown University medical schools found.

They compared the brain volumes of 8-year-olds born prematurely to those born full-term. They found significantly smaller areas in the cerebral cortex, especially in parts responsible for reading, language, emotion and behavior.

The study supports other findings that show preterm boys struggle more than preterm girls with speech and language and do worse in school and socially.

“Smaller brain volume has never been specifically related to increased risk of adverse outcome in males as opposed to females,” said Dr. Allan Reiss, director of Stanford’s Psychiatry Neuroimaging Laboratory.

“This is a striking and significant developmental abnormality in males who were born preterm.”

The researchers used magnetic resonance imaging (MRI) to look at the brains of 65 preterm children - born at around 28 weeks gestation - and 31 healthy, full-term children.

Writing in the August issue of the Journal of Pediatrics, they said they found striking differences in gray matter - where signal processing and thinking happen - and in white matter - which includes the parts of the nerve cells that connect brain cells.

“In the preterm group as a whole, we found the volumes of both gray matter and white matter were reduced,” Reiss said.

“When we divided the preterm group by gender we found, bingo, the females had normal or preserved white matter volume, but the males’ volumes were reduced compared to their full-term peers.”

The damage is probably done because the lungs of premature babies are not well developed and they cannot get enough oxygen, Reiss said. Girls may be protected to some degree by hormones or by genes on the X chromosome.

“Girls have a real genetic advantage,” he said. “Unlike boys, they have two X chromosomes, giving them a layer of partial genetic redundancy that may help protect against the effects of preterm birth.”

Boys have one X and one Y chromosome.

It may be possible to develop treatments to prevent his damage, Reiss said.

“This research indicates that 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.