Since the 1970s, alarming discourses about declining fertility and the difficulties of balancing work and family have flourished in Western countries. Captured by the notion of the 'biological clock', they put women's reproductive age and the fertility decline to the centre of public and medical attention. Reproductive biomedicine constitutes a specific domain invested with hopes for technological and medical answers and a new market for fertility extension technologies, such as egg donation and social egg freezing. Addressing long-standing questions about the articulation of the biological and the social in the making of bodies and identities, this book questions the nature of reproductive ageing, a taken for granted 'fact of life' at the core of reproductive biomedicine. What is the biology of the 'biological clock' made of and how can we account for its embodied reality from a feminist perspective? Opening the black box of the biological, the book makes a way between essentialism and constructivism with the aim of accounting for its materiality, while also illuminating its political implications. By following the ontological choreographies of age-related infertility in the science and medicine of reproduction, this study explores how age materializes and documents what happens when reproduction meets ageing. Deeply transdisciplinary, it questions what is fixed about the biology of the fertility decline in a way which adds complexity to debates about the biomedicalization of reproductive ageing.