Motherhood is often positioned as incompatible with further education, and various cohort studies have revealed the many ways in which mothers are discouraged from and disadvantaged in higher education. Guided by role theory, we investigated the experiences of more than 1300 'PhD mums' from across the world as they simultaneously navigate the roles of doctoral researcher and mother (or mother-like role). Using a mixed-methods survey design, qualitative and quantitative results were analysed to reveal the contradictions and complexities of the PhD mum experience, with motherhood both straining and enhancing the doctoral journey. Motherhood may place considerable strains on doctoral researchers, including on their ability to conduct and write-up their research. These strains are exacerbated by inequitable and gendered role expectations, finite resources, and limited support, often at the expense of doctoral researchers' physical and mental well-being. However, it is not all negative, and PhD mums can bring a range of skills and attributes that are valuable to individual doctoral studies as well as doctoral programmes and institutions more broadly. The benefits also extend to the PhD mums themselves, their families, and their communities. This paper challenges unfounded assumptions about the commitment and ability of mothers to succeed in doctoral education, but also raises serious concerns about the role of institutions in perpetuating social inequalities while espousing commitment to diversity, equity, access, and inclusion.