We estimate the impact of a cash transfer to women on their (future) children’s birth outcomes, exploiting the introduction of a universal child benefit in Spain. Using administrative data from birth records and a regression discontinuity approach, we find that low-income women who received the benefit were much less likely to have low birth-weight children, several years down the road. A 2,500-euro transfer led to a 2.2 percentage point decline in low birth-weight status among women in poor households. Given that about 6% of children were born with low birth-weight, this represents a 36% reduction. We find no impact on gestational length, suggesting that the effect is due to faster intrauterine growth, possibly related to improved maternal nutrition (versus, for instance, lower stress). Previous evidence on the causal effect of cash transfers to poor families on child health and development has been mixed. Some recent research suggests that targeting pregnant women may be more effective than later interventions, given the strong persistence of fetal health effects. Our results suggest that the impact may be stronger if vulnerable women are targeted even earlier, before conception.