Lead author Dr. Lisa Smithers of the University of Adelaide's School of Population Health and colleagues analyzed data from more than 13,800 children who were born full-term.
The study, published in the journal Pediatrics, found babies who put on 40 percent of their birth weight in the first four weeks had an IQ 1.5 points higher by the time they were age 6, compared with babies who only put on 15 percent of their birth weight.
"Head circumference is an indicator of brain volume, so a greater increase in head circumference in a newborn baby suggests more rapid brain growth," Smithers said in a statement.
"Overall, newborn children who grew faster in the first four weeks had higher IQ scores later in life."
The study also found the babies with the biggest growth in head circumference also had the highest IQs.
"Those children who gained the most weight scored especially high on verbal IQ at age 6," Smithers said. "This may be because the neural structures for verbal IQ develop earlier in life, which means the rapid weight gain during that neonatal period could be having a direct cognitive benefit for the child."