Down's syndrome in babies may be caused by the mothers' exposure to radiation, a major German study has discovered.

Researchers from the Freie University in Berlin have discovered a direct link between the syndrome which suddenly increased six-fold in the city in January, 1987 and the Chernobyl nuclear reactor accident which happened nine months earlier.

These women were breathing in high levels of radiation especially iodine 131 for two weeks after the accident during which time they conceived.

The researchers were able to discount the usual theory that Down's syndrome is related to the age of the mother. In 1987, the average age of a mother with a Down's baby was 27.2 against the 10-year average of 27.4, and the percentage of mothers over 35 was 11.5 against the 10-year average of 11.2 per cent.

After making the discovery, they uncovered other studies which supported their conclusions. Incidents of Down's syndrome increased dramatically in Kerala, India and Yangjiang County, China after women were exposed to similarly high levels of background radiation from the soil.

The study group, led by Profesor Karl Sperling, accepts its evidence "contradicts current textbook opinion." Writing in the BMJ (16 July 1994), they say that any exposure to ionizing radiation, especially around conception, should be avoided.

The age of the mother per se would not seem a reliable indicator of Down's syndrome except that the older mother may have a higher build-up of radiation.

Their findings add evidence to the argument that Down's syndrome is a

result of environmental factors and not simply age. A major study in

1990 (The Lancet, 1990; 353:747-50) discovered that Down's syndrome babies had higher levels of aluminium in their brains.