We estimate the effect of family size on female labour supply using data from Poland and instrumenting for family size with twinning at first and second birth. To the best of our knowledge, this is the first study to causally address this relationship in a post‐communist country. We identify the average causal effect of an additional child on mother's employment to be −7.0 pp, but we do not find significant effects of additional children on female employment among families with two or more children. Furthermore, we identify a positive bias of OLS in the estimates among highly educated mothers and those from younger cohorts.