From New Orleans, at the e.hormone 2004 conference
To identify a young rodent's gender without doing an elaborate test, biologists measure the distance from the animal's anus to its genital opening. This anogenital distance is slightly, but reliably, longer in males than in females—unless those males were exposed in the womb to pollutants, such as phthalates, that can alter fetal sex-hormone production. When that happens, a male's anogenital distance can become more similar to that of a female. Preliminary data suggest a similar trend in boys whose mothers were exposed during pregnancy to elevated amounts of some phthalates.
Phthalates, chemicals used in making many cosmetics, plastics, and other products, have become ubiquitous pollutants.