Reducing childhood overweight and obesity is a major global public health challenge. Interventions aimed at individual behaviours, or at the family or school level have failed to show an impact. Causal evidence for the suggestion that predisposition begins with exposures in utero is lacking. It is also unclear whether any effect of early life exposures emerge at different ages, which is important when assessing the impact of any interventions. For example, there is evidence from within sibship analyses that exposure to gestational diabetes causally results in higher BMI in postnatal life, but this may not emerge until late childhood. For other exposures, for example socioeconomic inequalities or access to greenspace whether associations differ at different ages is unclear. The LifeCycle project has a unique opportunity to examine how upstream, developmental origins and other early life exposures might relate to childhood BMI and how any associations differ between early childhood and early adulthood. In this ‘proof of principle’ study we will look at select exposures that have evidence for some effect on childhood BMI (maternal and paternal education, area level deprivation, access to greenspace, gestational diabetes and preterm birth) and use repeat cross-sectional analyses at different ages from early childhood to early adulthood to identify magnitudes of associations of these exposures with BMI from childhood to adulthood.