This article explains the political and institutional factors that affect reproductive rights policies in Argentina and Mexico. Consistent with the comparative literature on gender, politics and institutions, the article reveals that federal arrangements define the arenas in which advocates can challenge governments. The comparative analysis suggests that the content and variations of policy outcomes are not only determined by the legal distribution of such rights. Women's positions towards abortion and contraception was greatly shaped by partisanship and ideology, and this was critical to legislative outcomes, while the number of women legislators was only important to introduce the issues. Important for further research are the effects of institutional and party instability on women's organising in legislatures, and the relevance of links between women's groups outside legislature and political parties for the success of gender equality policies.