But a baby's early rate of growth didn't influence the child's behavior later in life, according to the study.
"We found that faster growth in the first four weeks following birth was linked to a small increase in intelligence quotient scores at 6.5 years, but there were no clear effects on children's behavior," said the study's lead author, Lisa Smithers, a postdoctoral research fellow in early life nutrition at the University of Adelaide, in Australia.
She added that these findings suggest that "it is important that parents seek help for any concerns they might have about their baby's growth or feeding quite quickly so that any problems can be addressed early."
"[However], we cannot say that faster growth causes a higher IQ," Smithers said. "It is possible that a phenomenon called 'reverse causality' may be at play, for example, if children with lower IQs had poorer growth."
The study results appeared online June 17 and in the July print issue of the journal Pediatrics.
The study included about 17,000 mothers and their babies from Belarus. Only mothers who delivered a single, healthy baby were included in the study. In addition, the babies were all born at or after 37 weeks of gestation.
Researchers measured the babies' weights and head circumferences over the first four weeks of life. Intelligence was measured using several IQ scales that were combined to yield a full-scale IQ score at 6.5 years. The full-scale IQ scores can range from 50 to 150, Smithers said, and the average score is 100. To assess behavior, parents and teachers completed behavior questionnaires.
Babies with the highest growth in weight and head circumference scored 1.5 points higher on the IQ scale compared to babies with the lowest growth. The researchers found no statistically significant differences in children's later behavior based on early growth.
"Our study involved thousands of healthy babies, so our findings reflect a wide range of growth patterns that might be expected within a healthy population," Smithers said.
Researchers accounted for other important factors, such as family income and parental education, in their analysis.
"The size of the effect we found on children's IQ would not be noticeable to individuals," Smithers said.
But the results may be important in the bigger picture, a U.S. expert said.
"A 1.5-point difference would be meaningless in an individual child and that child's success in life, but on a population level, such a difference may matter," said Dr. Lisa Thornton, medical director of pediatric rehabilitation at LaRabida Children's Hospital in Chicago.
"It's clear, though, that brain growth equals [thinking ability] growth, and it's interesting to see that really early brain growth correlates to intelligence at 6 years," she said. "It shows that it's important that early feeding difficulties shouldn't linger."
Thornton said women who are having breast-feeding trouble should seek help sooner rather than later. "Breast milk is God's perfect food, but this study suggests that it's better to get nutrition early," Thornton said.
Both Thornton and Smithers said this study's findings don't suggest that parents should overfeed their babies.
"Babies should never be forced to eat," Smithers said. "Babies should be fed on demand. Overfeeding may raise other problems over the longer term, as there is some evidence to suggest that more rapid growth in infancy is linked to poorer health outcomes, such as obesity and high blood pressure. Our study draws attention to the importance of balance."
Thornton agreed. "Make sure the baby is getting enough food for optimal growth, but don't overfeed to try to make the baby smarter," she said.
To learn about overcoming breast-feeding problems that could interfere with early nutrition, visit the U.S. National Library of Medicine.