This article explores how to capture the dynamics of interactions on a forum, analyses the transformation of this space over time, and characterizes its organization. Combining qualitative and quantitative methods, it describes a large forum (170,000 messages) dedicated to prenatal screening, to gain insight into its dynamics and provide some sort of history of them. I analyse the organization and content of communication, taking into account the uneven distribution of participation within the forum and over time. I endeavour to understand how this discussion space has changed over time, and how a history is written both at individual level and on the level of the space as a whole. I conclude with reflection on the determinants of the dynamics observed, on the notion of community, and on appropriate methods for analysing these digital discussion spaces.