Background: Pregnancy is often considered to be a 'teachable moment' for health behaviour change. However, existing behavioural models (TM and COM-B) were not developed specifically for, nor widely tested, in this population. The overall aim of this thesis was to understand pregnancy as a teachable moment by developing a pregnancy-specific model of health behaviour change. Methods: Four studies were conducted. The first, a systematic review and meta-synthesis, investigated factors influencing health behaviour change during pregnancy. Subsequently, a theoretical mapping exercise was conducted to assess the extent to which the COM-B and TM models explain behaviour change during pregnancy (study two). The third study investigated longitudinally, which model best explained changes in eating behaviour during pregnancy. Finally, a qualitative interview study explored influences on women's eating behaviour throughout the antenatal period. The findings across the studies were synthesised to inform the development of a pregnancy-specific model of health behaviour change. Findings: The findings from the first study revealed a myriad of internal and external factors that influence antenatal health behaviour. In the second study, the COM-B and TM models were found to be limited when applied to the context of pregnancy. Findings from the third study revealed that both models explained more variance in eating behaviour in early- and late-pregnancy, compared to mid-pregnancy and the postnatal period, however neither adequately accounted for behavioural changes. Qualitative findings revealed a range of influences on women's eating behaviour throughout the antenatal period. Whilst pregnancy is often conceptualised as one teachable moment in-and-of-itself, mid-pregnancy was highlighted as providing a more salient opportunity for change than other stages. Based on these findings, a pregnancy-specific behavioural model was developed that reflects six key domains understood to influence health behaviour during this time. Conclusions: The COM-B and TM models fail to adequately explain pregnancy as a teachable moment for health behaviour change. Indeed, the conceptualisation of pregnancy as one teachable moment is flawed. The pregnancy-specific model provides an enhanced understanding of pregnancy as a teachable moment for behaviour change and highlights the nuanced factors influencing behaviour during this time. This understanding can be used to develop more appropriate interventions and to inform various aspects of clinical practice, to better support pregnant women and their infants.