This article focuses on the historical confrontation between Western obstetrical medicine and indigenous midwifery in nineteenth-century Siam (Thailand). Beginning with the campaign of medical missionaries to reform Siamese obstetrical care, it explores the types of arguments that were employed in the contest between these two forms of expert knowledge. Missionary–physicians used their anatomical knowledge to contest both particular indigenous obstetrical practices and more generalized notions concerning its moral and metaphysical foundations. At the same time, by appealing to the health and well-being of the consorts and children of the Siamese elite, they gained access to the intimate spaces of Siamese political life. The article contends that the medical missionary campaign intersected with imperial desires to make the sequestered spaces of Siamese political life more visible and accessible to Western scrutiny. It therefore reveals the imbrication of contests over obstetrical medicine and trade diplomacy in the imperial world.