This paper investigates the hidden yet persistent cost of conflict to birth weight outcomes for 53 developing countries experiencing conflict in the past three decades (1990-2018). Exploiting the variation across districts and conception months-years, we find that intrauterine exposure to armed conflict in the first trimester of pregnancy reduces child's weight at birth by 2.8% and raises the incidence of low birth weight by 3.2 percentage points. Infants born to poor and low educated mothers are especially vulnerable to the adverse repercussions of armed conflict. Given the long-lasting consequences of poor infant health over the life cycle, our findings call for global efforts in the prevention and mitigation of conflict. Extra attention should be directed to children and women from disadvantaged backgrounds.