Fertility strongly declined in the Nordic countries in the 2010s, and first births explained most of the decline. It is unclear how partnering-first birth dynamics have changed over time, and to what extent the decline is driven by lower fertility in unions and changes in partnering. Using full coverage Finnish register data and a Markov chain multistate approach we calculated the yearly age-specific transition probabilities across states of single, cohabitation, marriage, and first birth among 15-45-year old childless men and women living in Finland in 2000-2018. We found long-term declines in the transition to marriage among all educational groups, and short-term declines in the transition to cohabitation among the lower educated individuals. Declines in first births within unions were observed across both cohabiting and married couples and among all educational groups. Lower fertility in unions was more important than lower partnering rates in explaining the fertility decline in Finland.