Nutrition during pregnancy may affect IQ
This is probably because heavier babies have been better nourished in the womb at crucial stages of brain development.
Other studies have clearly shown that being underweight at birth seems to correspond with poorer mental development.
Premature babies are at a much greater risk of being born underweight.
However, the latest study, published in the British Medical Journal, suggests that the weight to intelligence link extends to normal-sized babies.
A team of researchers from the Center for Urban Epidemiologic Studies in New York looked at 3,484 babies born between 1959 and 1966.
Brothers and sisters were tested so that the effects of birthweight alone could be separated from the effects of different diets or other factors.
Up to 10 points
Their birthweights varied from 1.5kg (3lb 5oz) to just over 4kg (8lb 13oz) , and IQ was tested seven years later.
In general, higher birthweights meant slightly more IQ - the average difference between babies of less than 2.5kg (5lb 8oz) and those of up to 4kg (8lb 13oz) was 10 IQ points.
The study authors wrote: "Although the reported effects of variation within normal birth weight on IQ are modest and of no clinical importance for individual children, they could be important at a population level.
"In addition, these effects could shed light on links between foetal growth and brain development."
Other studies have revealed similar results - one in Denmark found that increasing birthweight kept on increasing IQ until the baby reached 4.2kg (9.24lbs).
Although not proven, it is suspected that the relationship between IQ and birthweight may be due the nutrition available to the foetus during early pregnancy, a time vital to neurodevelopment.