BACKGROUND: Becoming a parent is a time of both joy and stress. Associations between exposure to postnatal depression and negative child outcomes underscore the importance of understanding trajectories and correlates of perinatal depression and anxiety. METHODS: In a study of 438 expectant couples (from the UK, USA and Netherlands) tracked across four time-points (third trimester, 4, 14 and 24 months), we used dyadic latent growth curve modeling (LGCM) of self-reported symptoms of anxiety and depression (CES-D, GHQ, STAI) to investigate the affective impact of becoming a parent. RESULTS: Confirmatory factor analyses of anxious-depressive symptoms revealed a single latent factor with measurement invariance across time and parent. Dyadic LGCM intercepts showed greater prenatal problems in mothers compared with fathers. LGCM slopes revealed stable maternal problems but worsening paternal problems. Both intercepts and slopes showed significant within-couple associations. Controlling for prenatal salivary cortisol levels and perinatal couple relationship quality, support from friends attenuated mothers' psychological distress and support from family reduced fathers' psychological distress across the transition to parenthood. LIMITATIONS: Our sample was low risk (i.e., predominantly well-educated and affluent and no history of serious mental illness), limiting the generalizability of findings. In addition, the inverse association between psychological distress and social support may, in part, reflect the use of self-report for both measures. CONCLUSIONS: The international dyadic longitudinal design strengthens conclusions regarding variation in trajectories of psychological distress in both mothers and fathers. Crucially, social support appears pivotal in enabling new parents to flourish.