The cost of not investing in the next 1000 days: implications for policy and practice.

Building on the evidence from the first paper in this Series highlighting the fundamental importance of healthy and nurturing environments for children's growth and development in the next 1000 days (ages 2-5 years), this paper summarises the benefits and costs of key strategies to support children's development in this age range. The next 1000 days build on the family-based and health-sector based interventions provided in the first 1000 days and require broader multisectoral programming. Interventions that have been shown to be particularly effective in this age range are the provision of early childhood care and education (ECCE), parenting interventions, and cash transfers. We show that a minimum package of 1 year of ECCE for all children would cost on average less than 0·15% of low-income and middle-income countries' current gross domestic product. The societal cost of not implementing this package at a national and global level (ie, the cost of inaction) is large, with an estimated forgone benefit of 8-19 times the cost of investing in ECCE. We discuss implications of the overall evidence presented in this Series for policy and practice, highlighting the potential of ECCE programming in the next 1000 days as an intervention itself, as well as a platform to deliver developmental screening, growth monitoring, and additional locally required interventions. Providing nurturing care during this period is crucial for maintaining and further boosting children's progress in the first 1000 days, and to allow children to reach optimal developmental trajectories from a socioecological life-course perspective.