Diethylstilbestrol: The earliest synthetic form of the female hormone estrogen. Abbreviated DES. DES was widely prescribed between 1940 and 1971 to prevent miscarriages. The use of DES during pregnancy declined and was halted when it was found that, when given during the first 5 months of pregnancy, DES can interfere with the development of the fetal reproductive system. Women whose mothers were given DES during pregnancy are at increased risk for an uncommon form of cancer called clear cell adenocarcinoma of the vagina and cervix. Daughters of women who took DES also have an increased risk of having the most common forms of cancer of the cervix. They are also at increased risk of having anatomic abnormalities of the vagina, cervix, and uterus and of having tubal (ectopic) pregnancies, infertility, miscarriages, or premature births. Sons of women who took DES are predisposed to abnormalities of the testicles, such as abnormally small testes and failure of the testes to descend into the scrotum; these abnormalities increase the risk of testicular cancer. People who believe they may have been exposed to DES before birth should inform their physicians of their exposure so that they can be appropriately examined and monitored. DES is still available for prescription in the US for the palliative treatment of breast and prostate cancer.