Antagonists in the debate over whether the maternal stress response during pregnancy damages or culls fetuses have invoked the theory of selection in utero to support opposing positions. We describe how these opposing arguments arise from the same theory and offer a novel test to discriminate between them. Our test, rooted in reports from population endocrinology that human chorionic gonadotropin (hCG) signals fetal fitness, contributes not only to the debate over the fetal origins of illness, but also to the more basic literature concerned with whether and how natural selection in utero affects contemporary human populations.

We linked maternal serum hCG measurements from prenatal screening tests with data from the California Department of Public Health birth registry for the years 2001–2007. We used time series analysis to test the association between the number of live‐born male singletons and median hCG concentration among males in monthly gestational cohorts.

Among the 1.56 million gestations in our analysis, we find that median hCG levels among male survivors of monthly conception cohorts rise as the number of male survivors falls.

Elevated median hCG among relatively small male birth cohorts supports the theory of selection in utero and suggests that the maternal stress response culls cohorts in gestation by raising the fitness criterion for survival to birth. Am. J. Hum. Biol., 2012. � 2012 Wiley Periodicals, Inc.