How depressive symptoms unfold within a couple during the perinatal events of pregnancy, childbirth, and early parenthood is poorly understood. In this prospective study, we aim to investigate the reciprocal relation between maternal and paternal depressive symptomatology, specifically how symptoms in 1 partner relate to subsequent symptom level changes in the other partner throughout the perinatal period. Further, we aim to identify parents who are particularly vulnerable to the development of disruptive processes of negative mood states. Data were collected from 1,036 mothers and 878 fathers participating in the Little in Norway study from midpregnancy until 12 months postpartum. Depressive symptoms were assessed at 7 time points (4 prenatally) in both parents. Partner-related attachment was measured early in pregnancy. By using an autoregressive latent trajectory modeling approach, accounting for time invariant confounding, we found mothers’ depressive symptoms late in pregnancy to predict elevated symptom levels in fathers 6 weeks after birth, with a small effect size. No other time-adjacent effects were observed among partners at other time points or with the opposite directionality. However, moderation analyses revealed that among parents characterized by insecure partner-attachment styles, additional cross-lagged pathways were evident during pregnancy and throughout the first year of parenthood. Clinicians need to be aware of fathers’ vulnerability to symptom development in instances of maternal perinatal depressive states at the time around childbirth, and tailor preventive and treatment efforts to address both parents’ needs. Further, particular attention should be directed to parents with heightened susceptibility to prolonged depression contagion processes.