Educational disparities in cohort fertility rates (CFR) often serve as the starting point for analyzing the socioeconomic and gendered drivers of fertility. As the majority of children are born to couples, the relationship between education and fertility is the compound of both partners’ education. Measuring both partners' educational levels is generally considered superior to measuring only one partner’s educational level. However, no study has comprehensively analyzed the role of the couple’s educational composition (educational pairing) in cohort fertility. This study uses Finnish register data to provide the first analysis of cohort fertility rates from the perspective of educational pairings across the life course. For the complete female birth cohorts born between 1969 and 1975, we decompose the CFR from births by the educational pairing of the union in which they occur. This novel measure complements previous work that has focused on the incidence or instantaneous birth risks by educational pairing, as it allows us to quantify lifetime fertility as the joint sum of lifetime exposure to unions of different educational pairings and the rate of childbearing within such unions. Our analysis reveals that completed cohort fertility in Finland— for the most recent cohorts from which such measures can be derived— is predominantly the result of births in educationally hypogamous or homogamous unions and unions where at least one partner has tertiary education. In particular, the CFR of highly educated women is composed of births to highly educated partners. We demonstrate that the CFR by educational pairing is a conceptually valid measure of macro-level population patterns as an aggregate measure of unions rather than individuals.