PREGNANT women who work in occupations that may bring them into contact with heavy metals are at more than double the usual risk of having a baby boy with hypospadias - a birth defect linked to hormone changes in which the opening of the penis is on the shaft instead of the tip.

The finding, by scientists from the Telethon Institute of Child Health Research in Perth, could shed light on the rise in the condition, which separate research has shown has increased by 50 per cent over the last two decades and now affects at least one in 118 boys in Western Australia.

Women who worked in dentistry, defence, laboratories and petrol stations were identified as possibly at risk. Hairdressers, beauticians and cleaners, who were exposed to a range of different chemicals, were also at a smaller increased risk - about 30 per cent higher than other mothers - of having a baby with the condition.

About 9 per cent of mothers of affected children had been in contact with potentially damaging chemicals through their work, versus 7 per cent of mothers of unaffected babies, according to the analysis based on state birth records, which was led by Natasha Nassar, a research fellow at the institute. She cautioned that the numbers were small, but noted that more severe hypospadias appeared to be linked to chemical exposure - especially pesticides, which doubled the chance of a severe case.

Pesticide and heavy metal exposure also increased the chance that hypospadias would be diagnosed together with other birth defects rather than in isolation, Dr Nassar wrote in her study, published in the journal Occupational and Environmental Medicine.

The chemical exposure of fathers made no difference to the chances their sons would be born with the defect, though electricians and men who worked with plastics or in factories were at elevated risk of having a son with the condition, which may result from excessive exposure to oestrogen in the uterus, or to the suppression of testosterone.

''This is the first study to show maternal exposure to heavy metals such as mercury, lead and cadmium were significant risk factors for hypospadias,'' Dr Nassar wrote.