Secondhand smoke exposure during pregnancy: Nicotine metabolites in maternal urine and breast milk as biomarkers of infant birth weight and length, a cross-sectional study

INTRODUCTION: Exposure to secondhand smoke (SHS) during pregnancy (P-SHS) has adverse effects on breast milk composition and infant birth outcomes. However, evidence clarifying the exact relationship between nicotine exposure and specific infant outcomes is currently limited. This study aims to explore the association between nicotine metabolite levels - in the urine and breast milk of mothers exposed to SHS - and infant birth weight and length, as well as to assess how P-SHS-related variables modify exposure risk. METHODS: This study was a cross-sectional study of first pregnancies from April to December 2025 in an urban low-income community in Bangkok, Thailand. This study used multi-stage purposive sampling to recruit pregnant women and their infants who were highly exposed to secondhand smoke. Nicotine metabolites were measured in mothers' third-trimester urine and post-delivery breast milk samples, and infants' birth weight and length were measured at the participants' households. All measurements were done after participants provided written consent and had answered the questionnaire. Multiple linear regression was used to assess the association between maternal nicotine metabolites and infant birth outcomes and between P-SHS exposure and participant nicotine levels. RESULTS: This study involved 95 pregnant participants exposed to household smoking and their 95 infants. Prenatal maternal urine nicotine metabolite level was significantly associated with breast milk nicotine concentration (p<0.001). Infant weight (p=0.048) and length (p=0.034) were significantly associated with prenatal maternal urine nicotine metabolite levels. Several P-SHS variables reflecting greater SHS exposure, such as smoking in the living room and bedroom (p<0.01), were significantly associated with higher nicotine metabolite levels in prenatal maternal urine. CONCLUSIONS: Prenatal P-SHS exposure elevated nicotine metabolite concentrations in maternal urine and breast milk, which were significantly associated with infant birth weight and length.