The effects of early life circumstances on cognitive skill formation are important for later human capital development, labor market outcomes and well-being. In this paper, we test the hypothesis that the first 1,000 days are the critical window for both cognitive skill formation and physical development by exploiting a randomized conditional cash transfer (CCT) program in Nicaragua. We find that boys exposed in utero and during the first 2 years of life, have better cognitive, but not physical, outcomes when they are 10 years old compared to those also exposed, but in their second year of life or later. These results confirm that interventions that improve nutrition and/or health during the first 1,000 days of life can have lasting positive impacts on cognitive development for children. The finding that the results differ for cognitive functioning and anthropometrics highlights the importance of explicitly considering cognitive tests, in addition to anthropometrics, when analyzing impacts on early childhood development.