This paper uses a cohort of 12,000 births to examine the effect of maternity rights on mothers' post-birth return to employment decisions. It uses a discrete hazard model to disentangle the effects of the terms of maternity rights entitlements from other factors that influence the timing of a mother's return to work. Mothers with rights have an underlying (but unobserved) stronger attachment to the labour market that prompts earlier return than on average. We take this into account by estimating a counterfactual distribution of return times using a sample of women who failed to qualify for maternity rights but who have similar levels of labour market attachment. Even when differential attachment is taken into account there remains a substantial impact of maternity rights on behaviour.