Objective This nested case-control study, based on the Shanghai Birth Cohort (SBC), aimed to explore the impact of early pregnancy exposure to organophosphorus flame retardants (OPFRs) on attention deficit hyperactive disorder (ADHD)-like symptoms in 4-year-old children, so as to provide epidemiological evidence regarding the health effects of emerging contaminant OPFRs in children.

Methods Strengths and Difficulties Questionnaire (SDQ) was used to assess ADHD like symptoms in 4-year-old children. Children with an SDQ hyperactivity subscale score ≥6 points were defined as cases, while those with a score <5 points were considered as controls. The case and control groups were matched at 1∶1 based on the child’s age (±6 months), sex, and parental or primary caregiver’s education level. A total of 105 cases and 112 controls were included eventually. Concentrations of eight OPFRs metabolites in early pregnancy urine samples were measured using ultra-high performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS), including di-phenyl phosphate (DPHP), di-m-cresylphosphate (DmCP), di-o-cresylphosphate (DoCP), di-p-cresylphosphate (DpCP), di-n-butyl phosphate (DnBP), di-iso-butyl phosphate (DiBP), bis(2-butoxyethyl) phosphate (BBOEP), and bis(2-ethylhexyl) phosphate (BEHP). Basic demographic information of mothers and children were collected through questionnaire surveys and medical records extraction. Binary logistic regression models were used to analyze the effect of individual OPFRs exposure during early pregnancy on ADHD-like symptoms, while a quantile g-computation (Qgcomp) regression model was employed to assess the effects of mixed OPFRs exposure (with detection rates >75%) on ADHD-like symptoms in 4-year-old children.

Results In this study, the detection rates of DPHP, DoCP, and the DmCP&DpCP in the urine of early pregnancy women were higher than 75%, with DPHP having the highest detection rate (86.18%). The median concentrations of DPHP were highest in both the case and control groups (0.396 μg·L^-1 and 0.305 μg·L^-1, respectively). Binary logistic regression analyses revealed that exposure to DPHP during early pregnancy increased the risk of ADHD-like symptoms in 4-year-old children (OR=1.262, 95%CI: 1.017‒1.565). The mixed exposure model analyses showed that early pregnancy co-exposure to OPFRs increased the risk of ADHD-like symptoms (OR=1.508, 95%CI: 1.012‒2.258), with DPHP being the primary contributor to the association.

Conclusion Early pregnancy exposure to DPHP is positively associated with an increased risk of ADHD-like symptoms in 4-year-old children. Additionally, DPHP contributed the most to the adverse effects of mixed OPFRs exposure on ADHD-like symptoms. However, these findings require further validation through other large-scale prospective cohort studies.