Cannabis Use During Pregnancy Was Linked to Smaller Babies | Research

The takeaway

In a Michigan pregnancy cohort, about 15% of participants used cannabis during pregnancy, and prenatal cannabis exposure was associated with smaller birth size after adjusting for other factors.

Pregnant individuals, OB-GYNs, midwives, and prenatal health educators.

What the researchers found

Covariate-adjusted models revealed a significant association between prenatal cannabis exposure and reduced birth size (β = -0.3; 95% CI: -0.5, -0.003), using a continuous birth size measure rather than just birth weight extremes.

Why it matters

With cannabis use during pregnancy increasing as legalization expands and risk perception decreases, understanding the effects on fetal development is critical for prenatal counseling and clinical guidelines.

The numbers in context

15% of participants reported cannabis use during pregnancy (95% CI: 12%–18%). Birth size association: β = -0.3 (95% CI: -0.5, -0.003). Sample: 584 participants from 23 Michigan clinics, recruited 2017–2022.

How the study worked

Prospective pregnancy cohort from the Michigan Archive for Research on Child Health linking prenatal survey data with birth records from 23 clinics (n=584). Generalized linear models adjusted for covariates including tobacco and alcohol use.

What this study cannot tell us

Moderate sample size (n=584). Cannabis use was self-reported, which may underestimate prevalence. Cannot isolate THC from other cannabis compounds. Observational design cannot prove causation. Single state (Michigan).

How to read the evidence

Prospective cohort design with covariate adjustment, but moderate sample size and self-reported exposure measurement.

When this study was published

Published 2025, data from 2017–2022.

The bigger picture

This study adds to evidence that prenatal cannabis exposure may affect fetal growth. Notably, it used a continuous birth size measure rather than just looking at extremes like low birth weight, potentially capturing subtler effects that previous studies missed.

Questions still open

What specific aspect of birth size is most affected — weight, length, or head circumference? Does the timing or frequency of cannabis use during pregnancy matter? Are effects mediated by THC crossing the placenta?

Common questions

How much smaller were babies exposed to cannabis?

The study found a statistically significant but modest reduction in overall birth size (β = -0.3). The effect was measured as a composite birth size score rather than specific grams of weight difference.

Is occasional cannabis use during pregnancy safe?

This study did not distinguish between frequency levels. Major medical organizations including ACOG recommend avoiding cannabis entirely during pregnancy, as THC crosses the placenta and interacts with fetal cannabinoid receptors.

Read the original research

Estimating the effects of prenatal cannabis exposure on birth outcomes.

The American journal on addictions, 34(1), 21-29

View on PubMed ↗Publisher’s paper ↗

Citation

Vanderziel, Alyssa; Anthony, James C; Barondess, David; Kerver, Jean M; Alshaarawy, Omayma. (2025). Estimating the effects of prenatal cannabis exposure on birth outcomes.. The American journal on addictions, 34(1), 21-29. https://doi.org/10.1111/ajad.13650