FRIDAY, Aug. 10, 2001 (HealthDayNews) -- Bigger babies are more likely to become smarter children, says new research.
Kids who weigh more at birth even babies in the normal weight range ultimately have a higher IQ, says a study in the Aug. 11 British Medical Journal.
The research was led by Dr. Thomas Matte in collaboration with the New York Academy of Medicine (NYAM) and Columbia University's Mailman School of Public Health, in New York City.
Matte, a senior epidemiologist at NYAM's Center for Urban Epidemiologic Studies, analyzed data from a study of 3,484 children born between 1959 and 1966 in 12 U.S. cities. The study was restricted to full-term babies, those born at 37 weeks gestation or later, and those who weighed between 3.3 pounds and nearly 9 pounds (less than 5.5 pounds is considered a low-birth weight.)
Each child received a full-scale IQ test at age 7.
The researchers say they ruled out factors such as race, education and socioeconomic status by including same-sex pairs of siblings.
By comparing each same-sex sibling pair, the researchers say they found birth weight was strongly linked with IQ in boys. Every 2-pound increase in birth weight was associated with a 4.6-point increase in IQ of boys.
However, the link was not as strong in pairs of girls, with every 2-pound increase associated with an IQ increase of only 2.8 points.
Moreover, the researchers found similar weight-IQ link in babies with both low and normal birth weights.
Matte says the difference of a few IQ points among individuals is not apparent. "That's not a difference that any normal person could tell by observation. The two children have a very similar potential for success in life," he says.
Parents of children in the normal birth weight range should not be concerned by slight variations between their kids, and they shouldn't make assumptions about which child might end up with greater IQ scores, he says.
But he says the effect of a few points matters more when considering an entire population. Besides concerns for health problems that cause low-birth weight, Matte says the study points out possible consequences for babies even in the so-called normal range. Even in that group, "there may be children who are not at the birth weight they should have been," says Matte.
Many more children than just those born with a strictly technical low-birth weight could be affected, he says.
"In terms of the population, there's no question that this is an important finding," says Naomi Breslau, the director of research in psychiatry at the Henry Ford Hospital in Detroit.
"Even in babies who are born above 2,500g [5.5 pounds] that is, they are in a normal range bigger is better," says Breslau.
But Matte says his study can't point to an ideal birth weight to make the smartest children since so many factors influence birth weight, including the mother's size, medical history or whether the child is firstborn.
"One of the implications of this work suggests that we need to not just look at whether you're low-birth weight or not, but to look at understanding the sources of variation in birth weight," says Matte.
Women who don't eat well or who smoke or drink during pregnancy are more likely to have low-birth weight children, and their children face a greater risk of chronic learning disabilities, visual problems and respiratory problems, he says.
While IQ scores can predict how well kids do in elementary and secondary schools, the connection between IQ scores and academic performance beyond high school is not as clear.
Some researchers have questioned the usefulness of IQ tests to predict success later in life, since the tests can't measure the social and personal skills required for successful employment.
However, Breslau says IQ scores from around age 7 are a strong predictor of IQ in later life.
"It definitely influences everything. IQ is a major predictor of how people do both in education and occupation. It's a very major predictor of achievement in life," she says.
What To Do: Read more about IQs from the Gale Encyclopedia of Psychology. You also can find out about low birth weight from The Future of Children. And Child & Family Canada provides an overview of low birth weight.
