Women have been warned to be careful about using bug spray while pregnant after a study linked the use of insect repellent to an increasingly common birth defect in boys.

Hypospadias is a condition where the opening of a boy’s urethra appears in the wrong place on the penis, usually on the underside rather than the tip.

It generally requires surgery.

It is estimated to affect about one in every 500 boys and seems to be on the increase, leading researchers to look at whether some chemicals in commonly used products are a contributing factor.

Signs of a link between exposure to insect repellents and a common congenital anomaly were reported by a team of epidemiologists in Occupational and Environmental Medicine.

“No previous study has linked hypospadias with insect repellents,” write Julien Dugas and colleagues, from research institutions in the UK and Spain.

Research in England last year found that women who came into regular contact with hairspray had a higher chance of having a boy with hypospadias.

The researchers have looked at the same group of women again, this time looking at their exposure to insect repellents and insecticides.

They found that women who gave birth to a son with the defect were more likely to have had used insect repellent in the early stages of pregnancy than those who gave birth to healthy babies.

The researchers also found that while no single factor such as living near farmland, using garden pesticides or using fly sprays or ant powder caused a statistically significant increase in risk those women who ticked more than one of these boxes was at slightly higher risk.

While the study didn’t specify the type of active ingredient used in the repellents, the chemical DEET (N,N-Diethyl-meta-toluamide) has been shown in previous studies to have harmful effects on pregnant rats.