Chemicals found in many everyday products can harm male reproductive development, research suggests.
Phthalates are found in soap
Phthalates are used in the manufacture of plastics, lubricants and solvents, and are found in cosmetics, medical equipment, toys, paints and packaging.
The University of Rochester team, New York, found exposure to the chemicals was linked to a higher risk of genital abnormalities in baby boys.
The study features in the journal Environmental Health Perspectives.
Previous research on animals has suggested phthalates may damage reproductive development by disrupting hormone levels.
PRODUCTS CONTAINING PHTHALATES
But until now evidence of a similar effect on humans has proved inconclusive.
The Rochester team, who examined 134 boys, found women with higher levels of phthalate-related chemicals in their blood were more likely to give birth to boys with undescended, or small testicles, small penises, or a shorter distance than usual between the genitals and anus.
It did not take exceptional levels of exposure to produce an effect - abnormalities were found in women exposed to levels below those found in a quarter of US women.
Lead researcher Professor Shanna Swan said: "We were able to show, even with our relatively small sample, that exposed boys were likely to display a cluster of genital changes."
Professor Richard Sharpe, of the UK Medical Research Council's Human Reproductive Science Unit in Edinburgh, said more work was needed to confirm the results.
But he told BBC News website: "It is significant. It is the first piece of evidence that we have that phthalates may cause adverse effects on reproductive development in human foetuses."
Professor Sharpe said the chemicals appeared to suppress production of the male sex hormone testosterone.
"Testosterone is absolutely critical to development - most of the things that make males different to females are down to pre-natal exposure to the hormone.
"It is not just the effect on genital development, but also on tissues throughout the body, including the brain."
The 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 of gender bending chemicals.
"At the moment regulation of the chemicals industry is woefully inadequate, and something needs to be done about this immediately."
The UK government is looking at how the regulation of hormone-disrupting chemicals could be made more effective under a new EU chemicals law.
Several types of phthalates have already been banned.