Mums-to-be who are exposed to insect repellent in the earliest phase of pregnancy are likely to give birth to boys with hypospadias in penises, say researchers.
This is where the opening of the penis is in the wrong place - usually back from the tip and on the underside - and it often requires corrective surgery.
According to University of NSW Professor of Toxicology and Occupational Health Chris Winder, the number of kids with birth defects has been increasing.
"This particular defect of the male urethra is quite common ... and has been linked to environmental sources as well as genetic problems," the Daily Telegraph quoted Winder as saying.
"... Here is more evidence that pregnant mothers, or mothers planning pregnancy, should limit their exposure to chemicals such as insect repellents," he added.
During the study, researchers quizzed mothers of 471 babies with hypospadias, and another 490 randomly selected babies, about their lifestyles and chemical exposures during pregnancy.
They found that use of insect repellents during the first three months of pregnancy was associated with an 81 per cent increased risk of hypospadias.
The mums did not state what type of repellent they used but their most common active ingredient is N,N-Diethyl-meta-toluamide - otherwise known as DEET.
However, authors have cautioned more research was needed before firm conclusions could be drawn.