This book tells a story about women who attained legendary status in the annals of medicine. They were exposed prenatally to what was promoted as a benign and exciting new wonder drug prescribed to millions of American women to prevent miscarriage from the 1940s to the 1970s. This new reproductive technology—the synthetic estrogen DES— proved to be ineffective in preventing miscarriage, and in the long run it has had profound and damaging consequences for children, especially daughters of the women for whom it was prescribed (Dieckmann et al. 1953; Giusti, Iwamoto, and Hatch 1995). In 1971, medical scientists observed an association between prenatal exposure to DES and a rare form of vaginal cancer (clear cell adenocarcinoma) in women under age twenty; using available medical categories, they identified this synthetic estrogen as the first “transplacental carcinogen” (Herbst, Ulfelder, and Poskanzer 1971). “DES Daughters,” as these women are now called, are also at risk for poor reproductive outcomes, including ectopic pregnancy, miscarriage, premature birth, and stillbirth (Giusti, Iwamoto, and Hatch 1995). Almost forty years later, DES-related cancer remains rare, but reproductive tract problems—including menstrual irregularities, poor reproductive outcomes, and structural or cellular anomalies—are common among DES Daughters.