Biologic tests detected nearly three times more prenatal cannabis use than self-reports | Research

The takeaway

In a large prospective US cohort, urine and umbilical cord testing detected 75-90% more prenatal cannabis use than self-reports, with younger age, being unmarried, government insurance, and high stress associated with underreporting.

Healthcare providers managing pregnancies, researchers studying prenatal cannabis effects, and pregnant individuals.

89.7% of prenatal cannabis use at delivery was detected only by cord testing, not self-report

What the researchers found

Among participants who used cannabis, 74.5% of use at mid-pregnancy and 79.6% at late pregnancy was detected only by urine assay, not self-report. At delivery, 89.7% was detected only by cord assay. Agreement between self-report and biospecimen was fair during pregnancy (Kappa 0.23-0.30) and slight at delivery (Kappa 0.11).

Why it matters

Research on prenatal cannabis effects relies heavily on self-reported use, but this study shows that self-reports miss the vast majority of actual use. This means virtually all epidemiological studies of cannabis and pregnancy outcomes likely underestimate exposure, potentially diluting observed associations.

The numbers in context

9,116 participants; 434 (4.76%) used cannabis by either measure. Self-report detected only 25.5% of mid-pregnancy use, 20.4% of late pregnancy use, and 10.3% of delivery use. Kappa agreement: 0.30 at visit 2, 0.23 at visit 3, 0.11 at delivery. Factors associated with discordance: age <30, unmarried, government insurance, moderate/high perceived stress.

How the study worked

Ancillary analysis of the nuMoM2b study, a US multicenter prospective cohort of 9,116 pregnant individuals (2010-2013). Self-reported cannabis use was assessed at three visits. Urine testing for THC metabolite (11-nor-9-carboxy-delta-9-THC) was performed at two visits, and umbilical cord testing at delivery. Agreement was evaluated with Kappa statistics; factors associated with discordance were assessed with multivariable logistic regression.

What this study cannot tell us

Data from 2010-2013 may not reflect current cannabis use patterns or attitudes post-legalization. Urine testing detects THC metabolites for days to weeks, so positive tests may reflect use before the person knew they were pregnant. The study population was first-time pregnant individuals, which may differ from multiparous populations.

How to read the evidence

Large multicenter prospective cohort with paired self-report and biologic assays at multiple time points, providing robust data on measurement agreement.

When this study was published

Published in 2025, using cohort data from 2010-2013.

The bigger picture

The worsening agreement over time (from fair to slight) suggests that as pregnancy progresses, social pressure to deny cannabis use increases. The near-complete reliance on biologic detection at delivery (89.7%) means self-report is essentially useless as a screening tool at that point. This has major implications for clinical screening protocols.

Questions still open

Has the gap between self-report and biologic detection changed since cannabis legalization? Would universal biologic screening during pregnancy be acceptable and feasible? Are the studies showing minimal prenatal cannabis effects actually measuring much lower exposure than assumed?

Common questions

How much cannabis use did self-reports miss?

Self-reports missed about 75% of cannabis use at mid-pregnancy, 80% at late pregnancy, and 90% at delivery compared to biologic testing.

Why do pregnant people underreport cannabis use?

Social stigma, fear of legal or child protective consequences, and healthcare provider disapproval likely contribute. The study found younger age, being unmarried, government insurance, and high perceived stress were associated with underreporting.

Does this mean other studies on prenatal cannabis are wrong?

Not necessarily wrong, but likely underestimating the true prevalence of exposure. If studies classified many actual users as non-users, the real associations between cannabis and pregnancy outcomes could be stronger than reported.

Read the original research

Self-Reported and Biologic Assessments of Prenatal Cannabis Use: Ancillary Analysis of a Prospective Observational Cohort.

Substance use & addiction journal, 29767342251389751

View on PubMed ↗Publisher’s paper ↗

Citation

Devlin, Paulina M; Allshouse, Amanda A; McMillin, Gwen; Chung, Judith H; Grobman, William A; Haas, David M; Pippen, Jessica L; Parry, Samuel; Reddy, Uma M; Saade, George R; Simhan, Hyagriv N; Silver, Robert M; Metz, Torri D. (2025). Self-Reported and Biologic Assessments of Prenatal Cannabis Use: Ancillary Analysis of a Prospective Observational Cohort.. Substance use & addiction journal, 29767342251389751. https://doi.org/10.1177/29767342251389751