Cohort studies which recruit women during pregnancy or at birth and follow up their offspring over the lifecourse provide the opportunity to investigate intra-uterine and early-life exposures in relation to developmental characteristics and later-life outcomes. However, due to confounding and other limitations, identification of true causal risk factors has proved challenging. In the context of several large, population-based birth cohorts, I have applied a series of causal inference methods which aim to minimise problems afflicting observational epidemiology and provide greater insights into modifiable early-life risk factors. In this thesis, I systematically assess the different methods in turn within the context of each birth cohort and the scientific question being addressed.