The children were monitored from birth

Those children who were more than 56cm long when born were six times more likely to show signs of asthma when they reached their teens than those measuring 50cm to 55cm.

Children with heads larger than 36cm in circumference at birth were three times more likely to have increased levels of an immune system substance that is involved in allergic responses by the time they were 11.

Birth weight also had an influence, with babies weighing less than 3kg at a reduced risk of allergies and asthma.

The researchers could not explain why bigger babies should be at greater risk.

However, one possibility they consider is that better nutrition for the mother leads to bigger babies, and this might explain why the incidence of the disease has increased in the developed world as living conditions have improved.

Improved living standards

Publishing their findings in the British Medical Journal publication Thorax, they said: "Our data suggest that relative undernutrition may be protective while overnutrition increases the risk for asthma."

Incidence of asthma has been increasing

Dr Mike D'Souza, a GP in Kingston and a member of the National Asthma Taskforce, said it was an intriguing finding - although difficult to explain.

He has studied birth dates to see what allergens the mother would have been exposed to at different stages of the pregnancy and how that related to incidence of asthma and allergies in children.

He thought a similar mechanism might be at work here.

"It's very tied in with the mother's diet - whether the mother is getting good nutrition in the final stages of pregnancy when the baby puts on huge amounts of weight."

If these foods contained allergens it might predispose the child to allergy, whereas exposure early on in the pregnancy would offer a protective effect, he said.

Historical answer

However, an alternative explanation might lie further back in our evolutionary past.

Allergies are thought to have served some useful function - such as ensuring the body could respond quickly to attack from parasites - and as such were a sign of good health at an earlier stage in humankind's development, Dr D'Souza said.

As such, a well-nourished baby - being a healthy baby - would also be prone to develop allergies.

"There are all these mechanisms that must have had some function in the past, but in the modern world nobody knows what that was."

A spokesman for the National Asthma Campaign said: "The researchers focus on children's size which contributes to our position that the contents of food - certain additives and so on - influence a child's likelihood of developing asthma and other allergies."