Pregnancy loss is a primary limit on human reproduction and a key driver of population dynamics. It shapes the composition of families and communities. It is also very difficult to observe. Drawing on new forms of online activity data from an app that tracks fertility over time, we demonstrate that U.S. pregnancy survival is socially patterned along multiple dimensions. We argue that understanding this process is of broad interest; it is essential to answering a number of central questions in the social sciences. We discuss two examples: (1) how early-life experiences shape later-life welfare and (2) how children’s traits affect their parents’ outcomes. We also discuss why these estimates have implications for ongoing changes in the reproductive health care landscape. With this approach, we extend a long history of demographic research on cohort selection to the prenatal period.