Emotion regulation (ER) is essential in understanding how the parent-child relationship influences a child's psychological development. This study examines how early interactions with parents, especially fathers, shape infants' ER abilities observable at a physiological level. While research has mainly focused on mother-infant relationships, the impact of father-infant interactions on neonatal physiological regulation is understudied. Parenting behaviors play a crucial role in early interactions, with prenatal depressive symptoms and shared time influencing these behaviors and relationships. However, most findings are based on mothers, overlooking fathers' influence. 119 families were recruited during pregnancy and invited to a laboratory at the University of Geneva three months postpartum. Each parent interacted with the infant separately while interactions were recorded, and interactive synchrony assessments were conducted to measure the quality of interactions. During interactions, an electrocardiogram (ECG) was also recorded in the infant to measure vagal tone, an indicator of ER abilities. Furthermore, parents completed questionnaires before birth and in the four weeks following the meeting in the laboratory. Overall, the results of this study confirm the impact of mother-infant interactions on the infant's physiological regulation. Fathers do not directly influence infants' regulatory processes, but an increased time spent with the infant in the mother's presence heightens the likelihood of direct influences. Fathers may follow an indirect pathway of influence, as they indirectly impact mother-infant interactions and infants’ regulatory processes during interactions with the mother. The results showed that mutual influences between family members play a role in infants’ regulatory processes as they might exacerbate the negative impact of parental depressive symptoms during the prenatal period on parent-infant interactions. This study highlights the importance of considering family interactions beyond the mother-infant dyad in understanding infant ER development. It provides insight into fathers' nuanced role in shaping infant ER and family dynamics, enhancing our understanding of early socioemotional development.