The role of the Web in political exchange has been crucial for society. Its platforms have connected people and allowed manifestation, organization, and access to information; however, they have also produced negative outcomes, such as increased polarization and fast disinformation spreading. These types of phenomena are not completely understood in the context of continuous technological change. Here we propose to grow knowledge in these issues by focusing on representativeness, through the following question: How demographic groups are represented in the discussion on micro-blogging platforms? Our aim is to answer this question on the discussion about a specific topic, abortion, as observed on one of the most popular micro-blogging platforms. As a case study, we followed the abortion discussion on Twitter in two Spanish-speaking countries from 2015 to 2018. Our results indicate differences in representativeness with respect to country, stance, and time of publication, a process that affects to on-going legislation. These findings show that demographic groups differ in how they generate content, and that under- and over-represented groups are not the same between countries, implying that single-country outcomes are not generalizable.