A baby who has not achieved full birth weight potential may also have not reached full brain development potential

Dr Marcus Richards, Medical Research Council

Researchers from the Medical Research Council based at University College London examined the link between birth weight and mental ability in 3,900 babies born in 1946.

As part of a project called the National Survey of Health and Development, the babies subsequently underwent tests of their mental ability at the ages of eight, 11, 15, 26 and 43.

The tests carried out during childhood concentrated on reading ability, arithmetic skills and non-verbal reasoning.

Those carried out in adulthood focused on areas such as memory, speed and concentration.

The researchers also examined the association between birth weight and educational attainment.

They found that heavier babies were more likely to score better on tests of their mental ability right up to the age of 26.

This effect still held good even after other factors such as sex and social status had been taken into account.

Heavier babies were also more likely to achieve higher academic qualifications.

Weakens with age

However, the link weakened with time and appeared to have vanished by the time tests were carried out at the age of 43.

Lead researcher Dr Marcus Richards said: "It seems clear that birth weight has an influence on cognitive function.

"A baby who has not achieved full birth weight potential may also have not reached full brain development potential. "

Dr Richards said there was evidence that factors such as maternal diet, alcohol use and smoking have an effect on birth weight.

However, he said the influence of these factors on mental development is unclear.

The research is published in the British Medical Journal.