Decisions about whether and for how long to breastfeed are shaped by mothers’ ability to combine care with paid work under institutional and workplace constraints. While breastfeeding provides welldocumented benefits for mothers and children, continuation after return to work may be difficult. We develop a formal economic model of breastfeeding and work decisions, accounting for physical, social and workplace constraints and document the role of mother’s return to work in determining breastfeeding behaviour using data from the UK Household longitudinal study (UKHLS). We employ an event study methodology to study breastfeeding behaviour around the time a mother returns to work. Accounting for differential timing of return to work by child age, we find that return to work leads to a 9.6 percentage point reduction in the probability of continuing to breastfeed. Such effects are partly driven by mothers whose jobs do not allow flexible working or working from home, and those who face longer commuting times. We also document industry differences with the strongest effects in retail, education and health, which we show are driven more by workplace constraints than workplace attitudes. We discuss implications for workplace policies.