Since the advent of parenthood is a catalyst for increasing and long-lasting gender inequalities, this thesis investigates why heterosexual couples adopt traditional, gendered divisions of household labour (or otherwise) at this time. Explanations offered by the work-family literatures raise questions regarding how far couples ‘can’t’ or ‘won’t’ share and much research has explored the drivers of work-family divisions. However, few studies have looked in-depth at how these important decisions are made in practice. Using a quantitative recruitment survey and qualitative interviews with 25 heterosexual couples across the transition to parenthood, this thesis explores how first-time parents in the UK negotiate the division of parental leave, paid work and childcare. In particular, it asks how constraints and ‘preferences’ interact in this process. The thesis makes a key theoretical contribution to explanations for gendered household divisions of paid and unpaid work by demonstrating a reciprocal relationship between work-family preferences and constraints. In keeping with the findings of existing studies, constraints shaped desires - what has been referred to as ‘adaptive preferences’ - but at the same time individuals’ perceptions of and responses to constraint were varied and shaped by their ideals and priorities – something that has so far received less attention in the literature. In light of these findings, the thesis uses a framework of ‘motivation’ and the duality of structure and agency to understand decision making. Underpinning these findings are further contributions to: 1) explanations for lack of active negotiation and discussion among couples; 2) understandings of work-family preferences; and 3) interpretations of survey measures. These findings suggest avenues for improving gender equality at the transition to parenthood and have important implications for policy on parental leave and flexible working.