Variations in Birth Weight Related to Boys’ Early Cognitive Development

By B. J. Almond

Studies have long shown that low birth weights can have an impact on cognitive and motor development later in a child’s life. A new Rice University study also has found that normal birth weight—at least, among male infants—is related to how readily they focus on a visual stimulus, an ability that later may play a role in some attention deficit hyperactivity disorders.

James Dannemiller

In his study, published in Infant Behavior and Development, James Dannemiller, the Lynette S. Autrey Professor of Psychology, chose to focus on visual orienting as one of the earliest dimensions of infants’ typical development. “Visual orienting is the earliest and most-developed of our abilities to focus and plays a role in our cognitive development,” explains Dannemiller. “The fact that it is associated with something physiological, namely birth weight, suggests the further importance of prenatal development.”

To examine the relation between birth weight and visual orienting, Dannemiller studied a sample of 944 infants between 2 and 5 months old. Information about the babies’ birth weight and gestational age was collected from the parents.

“Infant boys who are heavier at birth, but within the normal weight range, are more likely to focus toward a visual stimulus,” Dannemiller says. “On the other hand, while girls visually orient on average as well as boys, their birth weights appear to have no connection to this type of attention.”

Visual orienting, according to Dannemiller, may be involved in some types of attention deficit hyperactivity disorder (ADHD). “While ADHD is often thought of as an inability to sustain attention,” he says, “it also may be the result of overactive orienting with anything in the environment grabbing the person’s attention.”

The discrepancy between genders in Dannemiller’s findings is similar to data reported from a study in England that saw a correlation between normal birth weight and intelligence measures in adult life. In that study, researchers also reported a stronger correlation for males than females.

Dannemiller also discovered that abnormally large male and female babies within his sample—those with birth weights of more than 10 pounds—showed less inclination to orient their eyes toward a visual stimulus. One of the causes of this condition, called “macrosomia,” is uncontrolled diabetes in mothers during pregnancy. None of the mothers of the 30 macrosomic infants in Dannemiller’s sample had this disorder, however. “I am gathering more details on the pregnancies of these mothers,” Dannemiller says, “to determine the reasons behind their babies’ high weights and why, in their cases, the ability to orient decreased for both male and female infants.”

Dannemiller cautions that the variation in birth weight associated with visual orienting is too small to be meaningful at the level of the individual child. As he explains it, if the orienting measure were analogous to an IQ test with a standard deviation of 15 points, then the effect size would equal three IQ points per kilogram of additional birth weight.

The study’s results, however, do offer clues as to how various prenatal factors within a population are related to behavioral development in infants and children.

“Disorders of attention can impair the child’s ability to learn,” Dannemiller says. “So it is important to understand how these processes develop during the early period when infants are just beginning to explore their worlds visually.”