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The rate of brain growth in the weeks before premature babies reach their predicted birth date is related to their cognitive abilities as children, researchers reported.

The rate of cerebral cortical growth between 24 and 44 weeks postmenstrual age (PMA) predicts complex cognitive functioning -- but not motor functioning -- in later childhood, according to David Edwards, DSc, of Imperial College in London, and colleagues.

The findings come from a study of 82 premature infants, who underwent magnetic resonance imaging of their brains up to eight times and had a battery of cognitive tests at about ages 2 and 6, Edwards and colleagues reported in the Oct. 18 issue of Neurology.

The bottom line, Edwards said in a statement, is that "in babies born preterm, the more the cerebral cortex grows early in life, the better children perform complex tasks when they reach 6 years old."

"The period before a full-term birth is critical for brain development," he added.

Edwards and colleagues reported data for 82 infants born before 30 weeks PMA, for whom they estimated growth in cortical surface area and cerebral volume from repeated brain MRI scans between 24 and 44 weeks PMA.

All told, they obtained 217 scans, including eight from one child, seven from three, and five from six babies. More than half the babies, however, were only scanned once or twice – two scans were obtained from 21 infants and one from 23.

The babies were "relatively normal" and did not have any focal brain lesions. In particular, there were no signs of cystic periventricular leukomalacia, hemorrhagic parenchymal infarction, or porencephalic cysts, Edwards and colleagues reported.

The babies' later neurocognitive abilities were tested at two years past their predicted birth date, using the Griffiths Mental Development Scale. At 6, the children's neurocognitive abilities were assessed by using the Wechsler Preschool and Primary Scale of Intelligence-Revised and the Developmental Neuropsychological Assessment; motor function was measured using the Movement Assessment Battery for Children.

Assessments were done on 62 children at age 2, 71 at age 6, and 51 at both ages, the investigators reported.

Overall, Edwards and colleagues found, average test scores "approximated population averages."

But, they reported, cortical growth was directly related to the Griffiths Developmental Quotient, the Wechsler test, and a summary score on the Developmental Neuropsychological Assessment. It was not related to scores on the movement assessment.

On average, the estimated reduction in cortical surface area at term-corrected age that was associated with a one standard deviation fall in test score was 7 on the Griffiths test, 6 on the Wechsler test, and 9.1 on the Developmental Neuropsychological Assessment.

The difference in cortical growth associated with changes in global ability was "large, consistent, and specific," Edwards and colleagues said – a one-standard deviation difference on the tests was associated with a difference of between 5% and 11% in cortical surface area.

On the other hand, there was no link between cortical growth and motor skills or between total brain volume and any test, they reported.

Edwards and colleagues said it remains unclear whether the links between cortical growth and neurocognitive function "reflect the (epi)genetic control of brain development or the environmental effects of preterm birth."

With some reservations, the study represents "significant progress toward understanding of mind-brain relations from a developmental perspective," according to Peter Rosenberger, MD, of Developmental Neurology in Boston, and Heather Adams, PhD, of the University of Rochester in Rochester, N.Y.

But among the caveats, they noted in an accompanying editorial, are the lack of a comparison group of infants born at term and the cross-sectional nature of the data -- even though the authors refer to the rate of cortical growth, many infants were only scanned once.

Also, they argued, the study "cries out" for better measures of cognition than the standard intelligence tests that were used.