In June 1805, Sacagawea fell gravely ill along the Missouri River during the outward journey of the Corps of Discovery. Historical discussion of her illness has failed to take into account the context of travel literature and writing at the time—in particular, the conventions governing references to personal experience, descriptions of Native American life, and the language of women's bodies. When William Clark and Meriwether Lewis wrote in their journals that Sacagawea was dangerously ill because she had "taken a cold," and that they blamed her partner Toussaint Charbonneau for her illness, they likely meant that she was pregnant and had miscarried. Their references, obscure now, made sense under the conventions of writing at the time and in the context of intense medical interest in the relationship of cold to women's menstrual cycles. Deciphering this episode adds to our understanding of the often intimate relations on the Lewis and Clark Expedition and broadens the ramifications of the concept of "cold" in the context of women's health. Such an interpretation brings new insight to this crucial chapter in U.S. western history and sheds light on the productive ambiguity often employed in discussions of women's health.