Lecture delivered by Florencia Torche, Assistant Professor of Sociology and Faculty Affiliate at the Steinhardt School of Education, NYU and Research Affiliate at INSPIRES, NYU School of Medicine. A growing literature highlights that in-utero conditions are consequential for individual outcomes throughout the life cycle, but research assessing causal processes is scarce. This paper examines the causal effect of one such condition (maternal stress) on one such outcomes (birth weight). Birth weight is a key outcomes because it has been shown to affect cognitive, educational, and socioeconomic attainment throughout the individual lifecycle. Using a major earthquake as a natural experiment and a difference in difference methodology, we show that maternal stress has a substantial detrimental effect on birth weight. This effect is focused on the first trimester of gestation, and it is mediated by reduced gestational age rather than intra-uterine growth restriction. Several robustness checks reject the hypothesis that the association is driven by unobserved selectivity of mothers. The findings highlight the relevance of understanding the early emergence of unequal opportunity and of investing in maternal wellbeing since the onset of pregnancy.