Sample size
We aimed to detect differences of 0.3SD between the intervention and the control groups. The preliminary data from the study area calculated the child development score of 67 in the cognitive domain. We assume the Intra-cluster Correlation Coefficient (ICC) of child development as 0.02, with the average number of pregnancies per cluster per year as 30, resulting in the design effect 1.58. Hence, to detect the desired improvement of 0.3SD in development score in the intervention group, with 95% confidence and 80% power, a total sample size of 452 mother–child dyads. Based on the previous experience, we accounted for a 20% loss to follow-up. Thus, the final sample size is 542 from 21 clusters, 271 in each group. However, from an ethical perspective, we enrolled all eligible participants fulfilling the inclusion criteria from the intervention and the control clusters in the study.