One Urine Sample May Not Capture a Pregnancy’s Full Chemical Exposure

A single urine sample collected during pregnancy has long been treated as a convenient snapshot of a woman’s exposure to a vast array of industrial and consumer chemicals. But a new study drawing on the National Institutes of Health’s Environmental influences on Child Health Outcomes (ECHO) Cohort suggests that for many of these substances, that snapshot may be little more than a fleeting glimpse. The research, published in the Journal of Exposure Science & Environmental Epidemiology, systematically measured how concentrations of non-persistent chemicals fluctuate within individual women across gestation, and its findings carry profound implications for how environmental health studies are designed, analyzed, and interpreted.

Non-persistent chemicals are those that do not linger in the body for months or years, unlike legacy pollutants such as persistent organic pollutants or heavy metals. Instead, they are metabolized and excreted within minutes, hours, or days. This category includes an enormous range of compounds that pregnant women encounter daily: phthalates from food packaging and personal care products, parabens from cosmetics, benzophenones from sunscreens, bisphenols from plastics, antimicrobials from soaps, pesticide residues, flame retardant metabolites, polycyclic aromatic hydrocarbons from combustion sources, melamine derivatives, and aromatic amines from tobacco smoke and dyes. Because these chemicals clear the body so rapidly, urinary biomarkers essentially reflect very recent exposure. Whether a single measurement can stand in for exposure over an entire pregnancy depends entirely on how stable an individual’s concentrations remain over months of gestation.

The research team, led by Giehae Choi of Johns Hopkins University together with colleagues from institutions across the United States, tackled this question by analyzing three urine samples collected at different time points during pregnancy from 90 participants at three ECHO Cohort sites: New York, Illinois, and Puerto Rico. The scale of the chemical analysis was remarkable. The laboratory measured 154 analytes representing 163 chemicals across 12 chemical classes, using liquid chromatography-tandem mass spectrometry methods at the Wadsworth Center Human Health Exposure Analysis Resource laboratory. Of these, 84 analytes were detected in at least 20 percent of samples at a given site, providing sufficient data for variability analysis.

The central statistical tool was the intraclass correlation coefficient, or ICC, which quantifies how reliably a single measurement represents a person’s typical level across repeated measurements. ICCs range from 0 to 1, with values approaching 1 indicating that concentrations are highly stable within an individual over time, and values near 0 indicating that measurements are essentially unpredictable from one time point to the next. The team calculated ICCs for 58 analytes with detection frequencies above 60 percent, using log2-transformed concentrations standardized for urinary dilution by specific gravity. They also computed kappa statistics for 56 analytes with more moderate detection rates, assessing whether the mere presence or absence of a chemical above the detection limit was consistent across the three pregnancy samples.

The results were strikingly heterogeneous. ICCs ranged from −0.12 to 0.91, and the pattern of high versus low reliability differed substantially by cohort site for most chemicals. Only two analytes bucked this trend: benzophenone-1 and propyl paraben showed consistently high ICCs above 0.5 at all three sites, indicating stable exposure throughout pregnancy, while bisphenol F showed consistently low ICCs below 0.1 everywhere. Beyond these exceptions, the site-specific patterns were revealing. All parabens showed high reliability in Illinois but lower reliability in Puerto Rico and New York. Benzophenones were more stable in New York. Antimicrobials were more stable in Puerto Rico. Most phthalate and polycyclic aromatic hydrocarbon metabolites were moderately to highly reliable in Illinois but less so in Puerto Rico, while most aromatic amines showed the opposite pattern, with moderate to high reliability in New York but lower reliability in Illinois.

One of the study’s most technically insightful contributions was its analysis of how concentration levels relative to detection limits shape reliability estimates. The researchers hypothesized that measurement error exerts a proportionally greater influence when concentrations sit close to the limit of detection, obscuring the true rank-ordering of individuals. They therefore calculated the ratio of each analyte’s site-specific median concentration to its detection limit. Chemicals with median concentrations more than 50 times the detection limit tended to show greater stability across pregnancy, with notable exceptions among certain phthalate metabolites. This exception is itself informative: high concentrations paired with low ICCs suggest that exposure to the source products occurs sporadically but intensely, rather than continuously. Conversely, chemicals with median concentrations within tenfold of the detection limit generally showed lower ICCs, indicating that low-level measurements may reflect recent, sporadic exposures rather than a chronic exposure state. Benzophenone-8 provided a fascinating exception, maintaining moderate ICCs despite low concentrations, which the authors interpret as evidence of near-continuous low-level exposure.

The consistently high ICCs for benzophenone-1 deserve particular attention given the widespread use of benzophenones as ultraviolet filters in sunscreens, cosmetics, and personal care products, as well as their use as UV stabilizers in food packaging. Because these compounds do not bioaccumulate and have short biological half-lives, stable urinary concentrations can only mean that exposure itself is nearly continuous throughout pregnancy. Benzophenone-1 may appear more stable than benzophenone-3 partly because it is a major metabolite of benzophenone-3, integrating exposure from multiple sources, and because only a small fraction of benzophenone-3 is excreted unchanged in urine. The finding that benzophenone-3 was stable in Puerto Rico and New York but not Illinois may reflect seasonal differences in sunscreen use, a reminder that exposure context shapes biomarker reliability.

The site-specific differences also point to the influence of demographic, geographic, and behavioral factors on exposure chronicity. The New York site, where a majority of participants were classified in the high cotinine category suggesting substantial tobacco smoke exposure, showed moderate to high ICCs for aromatic amines such as aniline, which is found in tobacco smoke. Continuous exposure to tobacco through active or passive smoking could plausibly explain this stability. Similarly, moderate ICCs for melamine analogs like ammelide in New York may reflect ongoing exposure through contaminated foodstuffs and water. These observations suggest that individual, cultural, and structural factors, rather than pure physiology alone, determine whether a person’s chemical exposures remain constant or fluctuate across the months of gestation.

The practical consequences for environmental epidemiology are substantial. For the majority of non-persistent chemicals examined, a single pregnancy urine sample appears to represent exposure near the time of collection rather than exposure across the entire pregnancy. This matters enormously because different gestational windows carry different vulnerabilities: organogenesis in the first trimester, neurodevelopment throughout gestation, and fetal growth in later trimesters may each be sensitive to chemical exposures at distinct time points. If researchers link a single third-trimester measurement to a birth outcome and interpret it as reflecting first-trimester exposure, they risk substantial exposure misclassification, typically biasing estimated health associations toward the null. The authors note that simulation work suggests a chemical with an ICC below 0.2 would require more than 35 samples per person to keep this bias below 10 percent, an approach that is neither affordable nor practical for most studies.

Fortunately, the study outlines realistic alternatives. Researchers can incorporate validation sub-studies with repeated measurements and apply measurement error correction techniques such as regression calibration or simulation extrapolation. They can time urine collection to align with known critical windows of developmental susceptibility rather than defaulting to arbitrary collection schedules. They can collect multiple samples and pool them within each subject, which reduces assay costs while improving the representation of typical exposure. Or they can turn to alternative biospecimens such as hair, which integrates exposure over longer periods. The study also provides some of the first evidence on within-pregnancy stability for understudied chemical classes, including halogenated phenols such as the carcinogen pentachlorophenol, melamine and its analogs, and aromatic amines, filling critical gaps in the exposure science literature. As chemical production and consumer patterns evolve, the authors caution that reliability estimates from earlier studies may not transfer to current exposures, making ongoing characterization of biomarker variability an essential, not optional, component of rigorous environmental health research. For now, the message is clear: when it comes to non-persistent chemicals in pregnancy, one sample tells you where a woman was this week, not where she has been for nine months.

Subject of Research: Intra-individual variability of urinary non-persistent chemical biomarkers measured at three time points during pregnancy

Article Title: Intra-individual variability of urinary non-persistent chemical concentrations during pregnancy in the ECHO Cohort

Article References: Choi, G., Xun, X., Bennett, D. H., Meeker, J. D., Sathyanarayana, S., Schantz, S. L., Trasande, L., Watkins, D. J., Pellizzari, E., Li, W., Kannan, K., Woodruff, T. J., Buckley, J. P., & for the ECHO Cohort Consortium (2026). Intra-individual variability of urinary non-persistent chemical concentrations during pregnancy in the ECHO Cohort. Journal of Exposure Science & Environmental Epidemiology. https://doi.org/10.1038/s41370-026-00979-z

Image Credits: AI Generated

DOI: 10.1038/s41370-026-00979-z

Keywords: ECHO Cohort, pregnancy, non-persistent chemicals, urinary biomarkers, intraclass correlation, phthalates, parabens, benzophenones, exposure assessment, environmental epidemiology, aromatic amines, melamine