Background: Organophosphate esters (OPEs) are ubiquitous flame-retardant replacements, yet longitudinal exposure patterns remain unclear; thus, we aimed to identify the determinants of early-life OPE exposure trajectories.

Methods: Urinary OPE metabolites were quantified in the Maternal-Infant Research on Environmental Chemicals (MIREC) cohort characterizing longitudinal exposure patterns. We measured concentrations of bis(2-chloroethyl) phosphate (BCEP), bis(1-chloro-2-propyl) phosphate (BCIPP), bis(1,3-dichloro-2-propyl) phosphate (BDCIPP), di-n-butyl phosphate (DNBP), and diphenyl phosphate (DPHP) at 6-13 and 16-21 weeks of gestation and at ages 2-5 and 7-9 years, adjusting for specific gravity. Temporal variability was assessed using random-intercept linear mixed-effects models with intraclass correlation coefficients (ICCs). We also used Spearman correlations to evaluate cross-time and cross-metabolite associations, latent class growth analysis (LCGA) to identify longitudinal exposure trajectories, and logistic regression to identify predictors of high-exposure trajectories.

Results: Urinary OPE levels were 1.8-14 times higher in childhood than during gestation, with prenatal reproducibility strongest for BCEP, BCIPP, and BDCIPP (ICCs: 0.63-0.70), moderate for DPHP (0.45-0.51), and low for DNBP (0.21-0.25), whereas childhood and prenatal-childhood ICCs were uniformly low (0.02-0.19). LCGA identified two distinct patterns per metabolite including a reference trajectory increasing from low concentrations and either a high-stable or early-peak prenatal trajectory. Predictors of the elevated trajectory included older maternal age, higher parity, smoking, female child sex, and lower household income.

Conclusion: Distinct trajectories and predictors revealed longitudinal variability and weak prenatal-childhood correlations in OPE exposures, highlighting the value of MIREC’s repeated childhood measurements for capturing dynamic exposure patterns.