This project focuses on the history of how teratogens, or agents which have the potential to cause birth defects, have been understood and tested for teratogenic potential in the US over the twentieth century. Prior to this time, teratogen studies were primarily concerned with cataloguing defects rather than exploring possible causes. At the turn of the twentieth century, experimental teratogen studies with the aim of elucidating mechanisms commenced. However, these early studies did not aim to discover human pregnancy outcomes and ways to prevent them, but simply focused on the results of exposing pregnant mammals to various physical and chemical insults.