Birthweight linked to IQ even among children of normal birthweight

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By Anthony J. Brown, MD

LONDON (Reuters Health) – Similar to the findings reported for children with low birthweight, IQ is also directly linked to birthweight among children with normal birthweights, according to a report published in the August 11th issue of the British Medical Journal.

"There is a lot of research showing that low birthweight has an adverse effect on intelligence," lead author Dr. Thomas D. Matte of the New York Academy of Medicine told Reuters Health. "More recently, there have been some studies indicating that this relationship continues into the normal birthweight range," he said.

"To confirm these findings and control for family social confounding factors that may have been present, we studied a population that included a large number of sibling pairs," he explained.

Dr. Matte and colleagues assessed the relationship between birthweight and IQ score at 7 years of age in 3484 children from 1683 families. All of the children were at least 37 weeks' gestational age at birth and weighed between 1500 g and 3999 g.

Linear regression analysis of one randomly selected sibling per family revealed a direct relationship between mean IQ and birthweight after controlling for maternal age, race, education, socioeconomic status and birth order, the researchers note.

Analysis of data from 812 male sibling pairs and from 871 female sibling pairs, revealed a significant direct association between birthweight and IQ for the males only. In fact, this significant association persisted even after adjusting for differences in birth order, maternal smoking, and head circumference and after limiting the analysis to children weighing 2500 g or more at birth.

"The reason for a significant effect in boys but not in girls is not known," Dr. Matte said. He theorized that "because boys are larger than girls at birth and also grow more in the latter part of pregnancy, they may be more sensitive to aspects of the uterine environment that restrict fetal growth." It may also have to do with "differences in prenatal brain development that are known to exist between boys and girls," he added.

BMJ 2001;323:310-314.

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