Research from biology and psychology has shown that the prenatal period is sensitive to the environment and critical for later development. While the effects of toxins such as alcohol and nicotine on the fetus are well documented, the effect of maternal stress is more difficult to assess. The main reason is unobserved selectivity. Women who experience or report high levels of stress may be different from those who don’t in ways that affect their pregnancies, making it impossible to disentangle the effect of stress from its common correlates. The question is important because stress is widespread, stratified along socioeconomic and racial lines, and may be a central mechanism for the noxious effect of poverty or discrimination on children. We examine the effect of maternal stress and address the unobserved selectivity problem in a recent ASR article.