Objective: The couple's relationship is affected by the transition to parenthood, but research findings are mixed, with some studies showing positive effects on the relationship and others indicating negative impacts. This study aims to explore first-time mothers' and their partners' perceived quality of dyadic relationship (QDR36) during pregnancy and the following eight years after childbirth as well as what factors that are associated with a higher QDR36. Methods: A prospective longitudinal cohort study on 917 Swedish parents, with repeated questionnaires at: gestational week 25 (T1); first week after childbirth (T2); six months (T3); one year (T4); two years (T5); four years (T6), and eight years (T7) after childbirth. Non-parametric tests and Cohen's effect size calculation were conducted to analyze change over time in QDR36. Multiple linear regressions were carried out with index for QDR36 as dependent variable, at all time points (T1-T7). Results: QDR36 varied over time among both first-time mothers and partners, showing an increase between T1 and T2, followed by a decrease between each point in time. QDR36 was statistically lower at T7 compared to all other points in time (T1-T6). Variations were observed in the variables that were associated with a higher QDR36 at different time points, however higher sense of coherence (SOC-13) and social support (MSPSS) were consistently associated. Conclusion: First-time mothers and partners' perceived quality of couple relationship declines the first eight years after birth. To support them in their couple relationship, healthcare professionals should acknowledge parents' sense of coherence and social support.