We study how maternal stress affects offspring outcomes. We find that in utero exposure to elevated levels of the stress hormone cortisol negatively affects offspring cognition, health, and educational attainment. These findings are based on comparisons between siblings that limit variation to short-lived shocks and controls for unobserved differences between mothers that could bias estimates. Our results are consistent with recent experimental results in the neurobiological literature linking exogenous exposure to stress hormones in utero with declines in offspring cognitive, behavioral, and motor development. Moreover, we find that not only are mothers with low levels of human capital characterized by higher and more variable cortisol levels but that the negative impact of elevated cortisol on their offspring is greater. These results suggest that maternal stress may play a role in the intergenerational persistence of poverty.