The multifactorial aspects of breastfeeding require measures at many levels to identify mothers in need of breastfeeding support from healthcare professionals. Our objective was to examine the relative importance of sociodemographic, pre/perinatal-, infant-, psychosocial-, and interaction-related factors affecting duration of exclusive breastfeeding. We used self-reported data from a community-based trial including 1265 women (response rate 49%) giving birth from January 2017 to February 2018. Data on outcome, duration of exclusive breastfeeding, were collected three and nine months postpartum; data on the study variables concerning known risk factors for breastfeeding cessation were collected two weeks postpartum. Crude and multiple Cox proportional hazards models were used for statistical analyses with additional analyses for time varying effects. Factors with an independent prognostic influence on duration of exclusive breastfeeding in fully adjusted models included early skin-to-skin contact (HR = 1.18 CI:1.04-1.33), intention to breastfeed (HR = 0.77 CI: 0.73-0.80), positive outcome evaluation, meaning the value mothers attributed to breastfeeding (HR = 1.33 CI: 1.08-1.63), higher level of self-efficacy (HR = 1.46 CI: 1.24-1.72), and maternal sense of security in relation to breastfeeding (HR = 1.31 CI: 1.14-1.50). Higher maternal BMI, lower self-efficacy, shorter breastfeeding duration of previous child, and hospitalization during birth were time dependent by affecting the exclusive breastfeeding duration primarily in the first months following birth. The results suggest that target groups in special need of early breastfeeding support are defined by being hospitalized, obese, having low self-efficacy or short previous breastfeeding experience. The extensive influence of psychosocial factors emphasizes the importance of including both practical facilitating guidance and positive verbal encouragement to ensure effective breastfeeding support.