In November 2011, the citizens of Mississippi voted down Proposition 26, a “personhood” measure that sought to establish separate constitutional rights for fertilized eggs, embryos, and fetuses. This proposition raised the question of whether such measures could be used as the basis for depriving pregnant women of their liberty through arrests or forced medical interventions. Over the past four decades, descriptions of selected subsets of arrests and forced interventions on pregnant women have been published. Such cases, however, have never been systematically identified and documented, nor has the basis for the deprivations of liberty been comprehensively examined. In this article we report on 413 cases from 1973 to 2005 in which a woman's pregnancy was a necessary factor leading to attempted and actual deprivations of a woman's physical liberty. First, we describe key characteristics of the cases and the women, including socioeconomic status and race. Second, we investigate the legal claims made to justify the arrests, detentions, and forced interventions. Third, we explore the role played by health care providers. We conclude by discussing the implications of our findings and the likely impact of personhood measures on pregnant women's liberty and on maternal, fetal, and child health.