To assess whether Family Integrated Care (FICare) in the neonatal intensive care unit improves maternal chronic physiological stress and child behavior at 18months of corrected age for infants born preterm. Follow-up of a multicenter, prospective cluster-randomized controlled trial comparing FICare and standard care of children born at <33weeks of gestation and parents, stratified by tertiary neonatal intensive care units, across Canada. Primary outcomes at 18months of corrected age were maternal stress hormones (cortisol, ie, hair cumulative cortisol [HCC], dehydroepiandrosterone [DHEA]) assayed from hair samples. Secondary outcomes included maternal reports of parenting stress, child behaviors (Internalizing, Externalizing, Dysregulation), and observer-rated caregiving behaviors. Outcomes were analyzed using multilevel modeling. We included 126 mother-child dyads from 12 sites (6 FICare sites, n=83; 6 standard care sites, n=43). FICare intervention significantly lowered maternal physiological stress as indicated by HCC (B=-0.22 [-0.41, -0.04]) and cortisol/DHEA ratio (B=-0.25 [-0.48, -0.02]), but not DHEA (B=0.01 [-0.11, 0.14]). Enrollment in FICare led to lower child Internalizing (B=-0.93 [-2.33, 0.02]) and Externalizing behavior T scores (B=-0.91 [-2.25, -0.01]) via improvements to maternal HCC (mediation). FICare buffered the negative effects of high maternal HCC on child Dysregulation T scores (B=-11.40 [-23.01, 0.21]; moderation). For mothers reporting high parenting stress at 18months, FICare was related to lower Dysregulation T scores via maternal HCC; moderated mediation=-0.17 (-0.41, -0.01). FICare has long-term beneficial effects for mother and child, attenuating maternal chronic physiological stress, and improving child behavior in toddlerhood. NCT01852695.