Simple SummaryPremature birth disrupts the continuity of bodily contact between mother and newborn, leading to greater prevalence of physiological, cognitive, and behavioral difficulties among prematurely born children. We followed a group of mothers and preterm infants who received skin-to-skin contact (Kangaroo Care, KC) in the neonatal period, as compared to a matched group who received standard incubator care, from birth to adulthood. Our aim was to investigate how a touch-based neonatal intervention shaped three outcomes in young adulthood: anxiety and depressive symptoms, levels of oxytocin, and markers of the immune system. Findings indicated that the effects of KC on adult outcomes were indirect and stemmed from its impact on mother, child, and the dyad across infancy, childhood, and adolescence. KC reduced maternal anxiety and depression in infancy, improved infant attention and child executive functions, and increased mother–child synchrony across development. These effects were interconnected and led to improved outcomes in adulthood. Our study uniquely describes mechanisms by which a birth intervention can influence adult outcomes through step-by-step enhancement and provides valuable insights into the topic of “developmental continuity”, a key controversy in developmental research.AbstractPremature birth disrupts the continuity of maternal–newborn bodily contact, which underpins the development of physiological and behavioral support systems. Utilizing a unique cohort of mother–preterm dyads who received skin-to-skin contact (Kangaroo Care, KC) versus controls, and following them to adulthood, we examined how a touch-based neonatal intervention impacts three adult outcomes; anxiety/depressive symptoms, oxytocin, and secretory immunoglobulin A (s-IgA), a biomarker of the immune system. Consistent with dynamic systems’ theory, we found that links from KC to adult outcomes were indirect, mediated by its effects on maternal mood, child attention and executive functions, and mother–child synchrony across development. These improvements shaped adult outcomes via three mechanisms; (a) “sensitive periods”, where the infancy improvement directly links with an outcome, for instance, infant attention linked with higher oxytocin and lower s-IgA; (b) “step-by-step continuity”, where the infancy improvement triggers iterative changes across development, gradually shaping an outcome; for instance, mother–infant synchrony was stable across development and predicted lower anxiety/depressive symptoms; and (c) “inclusive mutual-influences”, describing cross-time associations between maternal, child, and dyadic factors; for instance, from maternal mood to child executive functions and back. Findings highlight the long-term impact of a birth intervention across development and provide valuable insights on the mechanisms of “developmental continuity”, among the key topics in developmental research.