This study examines family formation trajectories as a manifestation of the second demographic transition (SDT) in three countries, comparing and contrasting two post-socialist countries (Romania and the Russian Federation) with France as benchmark country advanced in the SDT. By examining combines partnership and fertility sequences and transcending the mainly descriptive nature of trajectory-based studies, the current study expands our knowledge by including key explanatory factors, such as cohort, country, and educational level. Pooled data from the Gender and Generations Survey (N = 30,197) is used to engage in sequence, optimal matching (OM), cluster and multinomial logistic regression analysis. Post-Communist cohorts are significantly more likely to engage in long-term cohabitation, childbearing within cohabitation or lone parenthood. Educational level operates differently across countries, with the highly educated in Romania and the Russian Federation less likely to follow certain de-standardized paths. Non-marital cohabitation with children is associated with lower education in all countries. Strong differences emerge between the shape and stages of the SDT in Romania and Russia, with Russians having a higher probability to experience childbearing within cohabitation, opposed to Romanians who follow childless marriage patterns or adopt postponement and singlehood. The three countries differ in their advancement in the SDT and factors shaping partnering and childbearing choices. We conclude that although the SDT remains a useful construct, it needs to be supplemented with more nuanced contextual accounts of socio-economic conditions.