Pregnant women have been warned against using hairspray, as new evidence suggests that chemicals they contain may be linked to an increasingly common birth defect in boys.
Paediatricians have suggested that using sprays - and also some colouring shampoos - could raise the risk of the genital condition hypospadias.
The alert comes after a study showed that mothers who had used hair cosmetics during pregnancy were nearly twice as likely to have a boy with the condition.
One theory is that chemicals in the products disrupt male hormones and interfere with the development of the male genitalia in the crucial first three months of the pregnancy.
"This is the first study to demonstrate a link between maternal household exposure to these two hair cosmetics during early pregnancy and the incidence of hypospadias," say the researchers, whose study is published in the International Journal Of Environmental Research And Public Health.
"The precautionary principle should apply to pregnant women and they should be advised to limit their use of hair cosmetic."
Hypospadias affects about one in 250 men, often blighting relationships and causing lifelong physical and emotional trauma.
One of the main features is that the urine opening - called the meatus - emerges on the shaft or even the base of the penis, instead of at the tip.
The condition is also linked with undescended testes and fertility problems.
About 1,500 operations to correct hypospadias are carried out each year and experts claim the incidence is increasing, with the number affected more than doubling in a generation.
The team of doctors who carried out the research say that exposure to endocrine-disrupting chemicals in the womb could adversely affect the development of genitalia during early pregnancy.
The study at Amiens University Hospital in France involved comparing the use of hair cosmetics, chemicals and pesticides in 250 women who had given birth to boys with and without hypospadias. No association was found between hypospadias and the use of chemicals such as paint, solvents, gasoline, ink, glue and household products, but the use of hair cosmetic was found to raise the risk by 80 per cent.
Support for the theory comes from previous research.
One study reported a significant association between maternal occupational exposure to hairsprays in manufacturing plants and risk of hypospadias, while a second found an increased risk of newborns with hypospadias in women hairdressers.
Paul Anderson, consultant urologist at the Dudley Group NHS Foundation Trust who specialises in genito-urethral reconstructive surgery, including adults with hypospadias, said: 'The causes of the condition are not known, but hormones are very obviously involved, and the theory suggested in this new research is very plausible.
"At a very early stage in the pregnancy, the urethra is flat but at a critical point in development, it becomes a tube.
"If the hormonal mix in the womb is not right, that development may not happen or may be abnormal."