ObjectiveGiven that infant development is influenced by caregiver mental health, we tested whether an intervention to reduce antenatal anxiety could affect infant development. A secondary aim was to test depressive symptoms, maternal responsiveness, and maternal infant bonding as mediators of this relationship. MethodsBetween 2020 and 2022, pregnant women participated in a randomized controlled trial of the Happy Mother-Healthy Baby (HMHB) program based on cognitive behavioral therapy. We collected data on child development from 202 intervention and 198 control participants in a public hospital in Pakistan. Child development was measured using the Ages and Stages Questionnaires-Version 3 at six weeks postpartum. Using intent-to-treat analyses, we examined whether the intervention was associated with performance on the five ASQ-3 domains. Causal mediation analysis was used to assess depressive symptoms, bonding, and maternal-infant responsiveness as mediators. ResultsSocio-demographic characteristics were evenly distributed between study arms. Intervention arm infants showed a 2.1-point increase (95% CI: 0.12, 4.17) in communication scores compared to controls. Though not achieving statistical significance, intervention infants also showed a 2.0-point increase (95% CI: −0.06, 4.09) in gross motor development performance. Bonding, depression, and responsiveness were mediators between the intervention and infant communication (Bindirect=1.94 (95% CI: 0.86, 3.25) depression; Bindirect= 0.57 (95% CI: 0.09, 1.16) bonding; Bindirect= 0.53 (95% CI: 0.01, 1.21) responsiveness; and Bindirect= 1.94 (95% CI: 0.86, 3.25)). Bonding, responsiveness, and depression mediated 25%, 23%, and 87% of the total association, respectively. ConclusionsHMHB positively affected infant communication at six-week follow-up. Larger studies with longer follow-up are needed to confirm and extend these findings. Trial registrationClinicalTrials.gov NCT03880032; https://clinicaltrials.gov/ct2/show/NCT03880032