Exposure to radiation including x-rays and other medical tests increases a woman's risk of giving birth to a Down's syndrome baby, new evidence has discovered.

As older women are more susceptible, it seems the key is the accumulated dosage of radiation over a lifetime that matters, rather than a single burst. Modern medicine accounts for about 10 per cent of the radiation we're exposed to. Another element is fallout from nuclear plants. The rest comes from background radiation, such as cosmic rays and ionizing materials.

The connection, often refuted by governments, was made by scientists exploring the rate of Down's births and tests at nuclear plants. They examined a community in Fylde in Lancashire, England and discovered that incidents of Down's births peaked in 1958 and 1962 to 1964, when there were higher levels of nuclear fallout. The pattern was also followed in 1957, when there was a fire at the nearby Windscale now Sellafield nuclear power station. Women over the age of 35 seemed to be most affected.

Although Down's syndrome was first identified in 1866 when natural radiation was the only kind to which people were exposed cases have increased steadily this century, even though it is estimated we are exposed to only 10 per cent more radiation today.

This suggests that any kind of radiation such as medical or nuclear, other than background, can prove to be the straw that breaks the camel's back (Journal of Epidemiology and Community Health 1995;49: 164-70).