Prenatal phthalate mixture exposure increases early childhood internalising problems via maternal oxidative stress.

Prenatal phthalate exposure has been linked to internalising problems in children. Maternal oxidative stress is a plausible biological mechanism underlying this association as it is induced by phthalates and is associated with early internalising problems, but this pathway has not yet been empirically tested. In the Barwon Infant Study, a population-based birth cohort of 1,074 Australian children, we investigated the potential mixture effect of prenatal phthalate exposure on maternal oxidative stress and assessed whether oxidative stress mediates the relationship between the prenatal phthalate mixture and early internalising problems. Concentrations of phthalate metabolites and nucleic acid oxidation biomarkers (8-hydroxy-2'-deoxyguanosine and 8-hydroxyguanosine) were measured in third-trimester maternal urine, and phthalate daily intakes estimated. Internalising problems were assessed at ages 2 and 4 years using the parent-report Child Behavior Checklist and Strengths and Difficulties Questionnaire, respectively. Applying weighted quantile sum regression with repeated holdout validation, we found that higher phthalate mixture exposure was associated with increased maternal oxidative stress (0.22 standard deviations per interquartile range increase in mixture, 95% CI: 0.13, 0.31), with dimethyl phthalate and diethyl phthalate as the top contributors. Bayesian kernel machine regression yielded comparable results. For all child outcomes, counterfactual mediation analyses across holdout datasets revealed evidence of mediating effects, suggesting that oxidative stress is a pathway through which prenatal phthalate mixture exposure increases early-childhood internalising problems. These findings highlight the heightened risk associated with co-exposure to multiple phthalates during pregnancy and the potential to mitigate this harm not only through improved chemical regulation but also by monitoring and reducing maternal oxidative stress.