Bigger babies 'grow up to be brighter'

Heavier grow up to be brighter children, a study published in today's British Medical Journal reveals.

The research suggests that childhood intelligence is directly related to how much babies weigh when they are born, even when they fall within the normal weight range for healthy infants.

Nearly 3,500 primary school children, most of whom had a normal birthweight of 2.5 kilograms (5lb 8oz) and above, were involved in the study by scientists from the New York Academy of Medicine.

The children were given a series of verbal and written IQ tests at the age of seven, and their intelligence was matched to their weight at birth. The findings show that birth weight has a bigger influence on the intelligence of boys than girls. A one-kilogram increase in birth weight related to a 4.6 point increase in IQ among boys and a 2.8 point increase in girls.

Many studies have shown that children who weighed under 2.5kg at birth have less than average intelligence at school. They include a British study of 3,900 babies which was published in the British Medical Journal in January. That research, led by Dr Richard Marcus, found that bigger babies did better at school and gained more qualifications.

The US study has shown that the link between IQ and birth weight extends into babies born within the normal weight range of 2.5kg to 3.5kg.

The epidemiologists say that the association remains even after taking into account factors such as the mother's age, education and socio-economic status.