In 1971 doctors at the Massachusetts General Hospital reported high rates of unusual cancers of the vagina in teenage girls. Researchers traced the problem to a medicine their mothers were given during pregnancy intended to help prevent miscarriage—a synthetic estrogen called diethylstilbestrol (DES). DES has also been linked to other health problems, ranging from vaginal and uterine malformations and immune problems in girls, to undescended testicles, sperm abnormalities, and possibly testicular cancer in boys exposed to DES before birth. DES is an example of an endocrine disruptor.

Endocrine disruptors are chemicals in our environment that interfere with hormones—natural chemical messengers that travel in the bloodstream and regulate many important physiological activities. Endocrine disruptors may be natural phytoestrogens (estrogenlike chemicals that are made by plants) or synthetic chemicals used in medications, dietary supplements, cosmetics, and household products. They may also show up in pollution. Some examples...