The period a child spends in the womb is a crucial developmental stage, which influences the child’s outcomes both at birth and later in life. Although recent research has extensively investigated this developmental stage, we contribute by exploring the effect of prenatal maternal stress on pregnancy outcomes (low birth weight and prematurity). Most importantly, we examine its heterogeneity by socio-economic status (SES). Using Spanish birth registers, we study the 2004 Madrid train bombings as an exogenous source of prenatal maternal stress. Adopting a difference-in-difference approach, we find that the exposure had a detrimental effect on pregnancy outcomes (low birth weight and prematurity). Most surprisingly, we find that the detrimental effect of the exposure is concentrated only among low-SES offspring. We hypothesized three possible mechanisms to explain this heterogeneity, which have broader implications for the transmission of status across generations.