Chemical timebomb; BABY BOYS DEFECT THREAT.
Phthalates are substances used in the manufacture of thousands of products, including plastics, lubricants and solvents.
They can be found in toys, medical equipment, paints, inks and vinyl flooring as well as hair sprays, deodorants, nail polish and perfumes.
Animal studies have suggested that phthalates are hormone disrupters that can affect reproductive development.
But until now the evidence of harm to humans has been inconclusive.
The new study, led by Professor Shanna Swan from New York's University of Rochester, claims to show a highly significant relationship between human exposure to phthalates and genital defects in infants.
Women with higher levels of phthalate break-down chemicals, or metabolites, in their urine were more likely to give birth to baby boys with undescended or small testicles and small penises.
Significantly, the phthalate metabolite concentrations needed were not exceptional.
A quarter of women in the US were found to have levels associated with abnormalities in infant boys.
The researchers have written in the journal Environmental Health Perspectives: "These data support the hypothesis that prenatal phthalate exposure at environmental levels can adversely affect male reproductive development in humans."
A total of 134 boys, aged two to 30 months, were examined for the study.
The scientists first measured the distance between the anus and penis (anogenital distance, or AGD), which in rodents has been found to be a sensitive marker for the effects of chemicals that disrupt male hormones.
A comparison measurement called anogenital index (AGI) was then obtained by dividing an infant's AGD by his weight in kilograms.
This was done to account for the fact that some babies were bigger than others. Boys in the bottom quarter of the AGI scale were classified as having a "short" AGI.
On average, the 24 boys who made up this group had an AGI 19 per cent shorter than expected.
They were also much more likely to have genital defects, including undescended testicles, a "small and indistinct" scrotum and smaller penis.
Metabolite concentrations of four common phthalates - Din-BP, Di-i-BP, DEP, and BBP-BP - were measured in urine samples taken from the babies' mothers while pregnant. Researchers found higher levels of the metabolites were associated with shorter AGIs. Of the 10 boys whose exposure to phthalates in the womb was classified as "high", all but one had a short AGI.
Conversely, of 11 boys with low exposure scores, only one had a short AGI.
The scientists said that metabolite concentrations for mothers of boys with short AGI were consistently higher than those of other mothers.
Dr Swan added: "We were able to show, even with our relatively small sample, that exposed boys were likely to display a cluster of genital changes."
Reproductive biology expert Professor Frederick von Saal, University of Missouri-Columbia in the US, said the male hormone testosterone had a direct effect on anogenital distance, causing it to be longer in males than in females.
Anything which altered AGD was also likely to alter testosterone levels in the womb, which would have a dramatic effect on a developing male foetus.
He said: "The consequence of this is never just one outcome. Every aspect of masculine identity is altered when you see this measure altered in experimental animals.
"What you would expect is a global change in the masculinisation of a male baby. If you see this, you are very likely to see change in every other aspect of masculinisation as well."
Conservation group WWF, which campaigns against harmful environmental chemicals, described the findings as "startling".
Gwynne Lyons, toxics advisor to WWF UK, said: "This research highlights the need for tougher controls.
"The [British] Government is looking at how the regulation of hormone-disrupting chemicals could be made more effective under new EU chemicals law (REACH, concerning registration, evaluation, authorisation and restriction of chemicals).
"But the chemicals industry is lobbying very hard to water-down this legislation.
"Political agreement on this legislation is not expected until later this year, so it remains to be seen whether the UK Government has got the guts to stand up to industry lobbying.
"If they don't, wildlife and baby-boys will be the losers."
CONCERN: Male genital abnormalities may be due to phthalates