LONDON (Reuters) - Twins may have an increased risk of developing autism which could be due to an unknown environmental trigger that occurs in the womb before birth, according to New Scientist magazine.

Two separate studies by researchers in the United States and Britain have found a higher incidence of autism among twins.

"Being born a twin may increase the risk of developing autism," the weekly magazine said on Wednesday.

It cited research by geneticist David Greenberg, of Columbia University in New York, which found the incidence of autism was up to 12 times higher in identical twins and four times greater in fraternal twins than in the general population.

Scientists at St George's Hospital Medical School in London found a similar pattern in another database of siblings with autism.

"Among 79 sibling pairs, they found nine pairs of identical twins, which is 14 times the level of the general population," the magazine said.

The British researchers did not find an increased risk for non-identical twins but they said it could be due to the small size of the study.

Some scientific studies have suggested a genetic link for autism but the magazine said the twin studies suggest an environmental trigger before birth may be involved.

"Identical twins can be more closely associated in the womb, lacking one or two membranes that separate fraternal twins. This may result in a fiercer competition for resources," the magazine added.

Autism is a developmental disorder. Symptoms usually begin at about 18-24 months and it is more common in boys than girls.

The cause is unknown but fears that it could be linked to the measles, mumps and rubella (MMR) vaccine have led to a drop in the number of children in Britain being given the triple vaccine.

Children with autism become withdrawn and self-absorbed. Often they are unable to communicate. A minority of sufferers, known as autistic savants, show remarkable artistic, musical or mathematical skills.