In recent years, a number of studies have shown that low birthweight babies have lower IQ test scores at school age. Now findings reported in the current issue of the British Medical Journal indicate that a link between size at birth and IQ also exists in normal birthweight children. According to the study, bigger babies go on to receive higher scores on childhood intelligence tests.

Thomas Matte of the New York Academy of Medicine and his colleagues probed the relationship between birthweight and measured intelligence at age seven in more than 3,000 children, most of who had normal birthweights. Even after accounting for maternal age, race, education, socioeconomic status and birth order, the team found a direct association between the two.

The relationship was especially pronounced in boys. For example, whereas a one-kilogram increase in birthweight was linked to a 4.6 increase in IQ score in boys, the same birthweight increase in girls related to only a 2.8 increase.

"Considerable efforts continue to be directed at understanding causes of low birthweight, its neurodevelopmental consequences, and the mechanisms linking the two," the authors write. "Evidence from this and other studies suggests that these efforts should be broadened to examine the influences on fetal growth among those with normal birthweight and the neurodevelopmental consequences of such variation."

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