This paper analyzes the impact of prenatal care on the likelihood of low birth weight and prematurity using panel data on births taking place between 1995 and 2008 in the largest maternity ward in Uruguay. The use of difference-Generalized Method of Moments estimation addresses biases due to time invariant unobserved heterogeneity and feedback effects from prior pregnancies. Our estimates are larger than those usually found for developed countries: an adequate use of prenatal care - as defined by the Kessner criterion - decreases the probability of low birth weight by half and the likelihood of a pre-term birth by 70%. Even when imposing less stringent requirements on the total number of prenatal controls, the improvements over birth outcomes are considerable. In addition to indicating the crucial role of prenatal care in the birth outcomes of low-income populations, our analysis highlights the importance of using econometric techniques that use the full distribution of pregnancies to estimate the effectiveness of prenatal care.