In the nineteenth century, American medical practitioners helped craft a new science of monsters. The term “monstrous birth” had long been applied to newborns with congenital conditions such as anencephaly. When practitioners redefined “monstrosity” in scientific terms, they claimed to be stripping away its fraught connotations. Instead, recast as a biological phenomenon, the monster gained new social and cultural salience. Monstrosity gave form to pervasive ideas about the meaning of racial difference, the fragility of racial order, and the peril of racial degeneration. Miriam Rich explores the history of monstrosity as a modern scientific category, tracing the practices that transformed newborn bodies into medical specimens across the nineteenth- and early twentieth-century United States. Monstrous Conceptions vividly examines experiences of pregnancy, birth, and postpartum care; the preparation and display of anatomical specimens; and the production and circulation of scientific knowledge. It shows how diverse laywomen and their families engaged with medical meaning making even as predominantly white, male practitioners increasingly sought to assert authority over reproduction. Rich also reveals how the nineteenth-century category of biological monstrosity helped lay the groundwork for the American eugenics movement—and contributed to ideas about deviant and defective bodies that still haunt us today. Shedding new light on intertwined historical conceptions of race, sex, and disability, Monstrous Conceptions illuminates how medical science produced enduring notions of human difference.