This paper studies the effects of an in utero program on birth outcomes for vulnerable pregnant women. We use information from the Buen Comienzo program, an initiative run by the local government of Medellín, the second-largest city of Colombia. To identify the effects, we obtain matching estimates using data from program participants and national birth statistics. We find that the program increased the birth weight of participant children by 0.09 and 0.23 standard deviations for boys and girls, respectively, and reduced the prevalence of low birth weight by 2.6 and 4.6 percentage points for boys and girls, respectively. In terms of size, the program reduces the incidence of being short by 3 and 4 percentage points, for boys and girls, respectively. The program also significantly reduced preterm births between 3 and 8 percentage points. We also provide evidence of the existence of heterogeneous effects depending on a mother’s exposure to the program and her frequency of attendance. Finally, an estimate of the cost-benefit ratio of the program suggests that the benefits could be two to six times the costs, respectively, for boys and girls born to participant mothers with early exposure to the program.