The ‘First Thousand Days’ refers to the period from conception to the child’s second birthday. It is increasingly gaining traction as a concept to guide public health policy. It is seen as a crucial window of opportunity for interventions that improve child and population health. This review outlines the origin and growth of the First Thousand Days concept, and the evidence behind it, particularly in the areas of brain development and cognition; mental and emotional health; nutrition and obesity; programming and economic benefits. The review then describes UK experience of use of the concept to inform policy, and a recent government inquiry that mandates more widespread implementation.

Summary Background The prevalence of obesity in childhood is of high concern, especially in deprived populations. We explored trends in obesity following the introduction of a citywide strategy focused on preschool children. Methods Analysis of obesity prevalence using the National Child Measurement Programme 2009 to 2017 for primary‐school children in Leeds using 5‐year aggregated data for Leeds, comparable cities, and England as a whole. Results Prevalence of obesity in Leeds for school entry children fell significantly (9.4% to 8.8%), whilst comparable cities (CC) and England as a whole showed no change (comparison of trends: P < 0.001 and P < 0.001). The reduction in Leeds was primarily in the most deprived (11.5% to 10.5%; trend comparison CC: P < 0.001, Eng: P < 0.001), but also amongst the affluent (6.8% to 6.0%; trend comparison CC: P = 0.087, Eng: P = 0.012). Prevalence in older children in Leeds was unchanged whilst it increased for comparable cities and England (trend comparison CC: P < 0.001, Eng: P < 0.001). In the deprived, obesity increased: Leeds by 1.4%; CC 1.3%, England 1% (trend comparison Eng: P = 0.004). In the affluent, obesity prevalence reduced more in Leeds than elsewhere: 2% in Leeds, 0.8% in CC, and 0.7% in England (trend comparison CC: P < 0.001, Eng: P ≤ 0.001). Conclusions There has been a notable decrease in the prevalence of obesity especially amongst the most disadvantaged children at entry to primary school in Leeds. How this was achieved merits in‐depth consideration.