BACKGROUND: Although the harmfulness of second-hand smoke (SHS) exposure to foetuses is well-established, literature reporting foetal outcomes in experimental studies is limited. This follow-up study on preventing SHS exposure among non-smoking Indonesian pregnant women at home was based on a randomised controlled trial involving the provision of comic booklets with stickers to couples. This trial examined differences in the birth outcomes of participating couples between the experimental and control groups, factors associated with paternal smoking behaviour, and association between birth outcomes and paternal-related outcomes. METHODS: In total, 197 neonates of 286 couples who participated in an original trial were included. This study compared birth outcomes between participating couples using a comic booklet with stickers to reduce SHS exposure at home during pregnancy as the intervention. Pearson Chi-square tests were conducted to investigate significant differences in neonate sexes between the experimental and control groups. Independent sample t-tests were used to check for significant differences in birth outcome data between the experimental and control groups. A multiple regression analysis was applied to test the correlation between paternal smoking behaviour and the birth outcomes. RESULTS: The gestational age in the experimental group was longer than the age in control group (mean difference = 0.373, Cohen's d = 0.291, 95% CI [0.010-0.57], p-value = 0.048). Pregnant women's avoidance of SHS strongly influenced paternal smoking behaviour in both the experimental group (b = 0.559, 95% CI [1.175-2.109], p-value <
0.001) and the control group (b = 0.429, 95% CI [0.675-1.567], p-value <
0.001). No associations were observed between birth and paternal behaviour outcomes. CONCLUSIONS: The neonates' gestational ages were greater in the experimental group than in the control group because of our intervention effect
pregnant women's avoidance of SHS strongly influenced paternal smoking behaviour in both groups. Thus, the comic booklet intervention for smoking fathers with non-smoking pregnant partners helped reduce the risk of foetal developmental disorders. Couple-based interventions should be actively integrated into health worker strategies to effectively mitigate second-hand smoke exposure among pregnant women. Trial registration This study was registered in the UMIN Clinical Trials Registry under the registration number UMIN000035423 (01/02/2019).