This paper studies the effectiveness of prenatal care on low-income women’s birth outcomes. We analyze all births between 1995 and 2011 in Uruguay’s largest public maternity ward. We use mother-specific first differences to circumvent biases due to time-invariant, unobserved heterogeneity and implement robustness checks that reduce concerns about time variant shocks and feedback effects. We find that adequate use of prenatal care, as defined by early initiation and at least 9 visits, decreases the probability of low birth weight by 6 percentage points and the probability of pre-term birth by 11 percentage points, and increases birth weight by 149 grams.