Caesarean section (C-section) rates continue to rise globally. Yet, there is little consensus about the key determinants of rising C-section rates and the sources of variation in C-section rates across the world. While C-sections can save lives when medically justified, unnecessary surgical procedures can be harmful for women and babies. We show that a state-wide law passed in São Paulo (Brazil), which increased women's autonomy to choose to deliver via C-section even when not medically necessary, is associated with a 3% increase in overall C-section rates. This association was driven by a 5% increase in primary C-sections, rather than repeated C-sections. Since the law emphasizes women's autonomy, these results are consistent with mothers' demand being an important contributor to high C-section rates in this context.