Genetic factors cannot exclusively explain the recent rapid increase in obesity; its aetiology is likely a multi-faceted and complex mix of genes and environment. A proposed mechanism is the establishment of epigenetic patterns early in development, known as developmental programming. One epigenetic modification, DNA methylation, can modulate gene expression and can be influenced by environment factors i.e. diet and lifestyle. Thereby there is potential for DNA methylation to be a mediating mechanism in a disease such as obesity. Life-course epidemiological data from three birth cohorts will be used to investigate if this epigenetic marker could mediate early life risk factors and obesity.