Cannabis Exposure in the Womb Was Linked to Smaller Head Size Starting in the Second Trimester

After controlling for confounders, cannabis exposure predicted significant deficits in birth weight and head circumference. Head circumference reductions were evident as early as the second-trimester anatomy ultrasound and persisted through birth, with an adjusted deficit of more than 14 percentile points. Overall length and bone length were not significantly affected after controlling for confounding.

Retrospective cohort comparing 109 cannabis-exposed pregnancies (identified via self-report and urine screens) to 171 non-substance-using controls. All women had prenatal visits between 2010-2020, anatomy ultrasounds at 18-24 weeks, and no alcohol, tobacco, or other drug use. Regression analyses controlled for significant confounders.

This study isolates cannabis effects from other substances by excluding tobacco, alcohol, and other drug users. The finding that head circumference deficits appear as early as the second trimester and grow by birth suggests cannabis may specifically affect fetal brain growth during a critical developmental window.

Head circumference is a proxy for brain volume. Finding deficits this early in pregnancy, with increasing magnitude through birth, raises questions about long-term neurodevelopmental consequences for cannabis-exposed children. This adds to a growing body of evidence suggesting prenatal cannabis is not benign.

Observational design cannot prove causation. Cannabis use identified by self-report and urine screens, which may miss or misclassify exposure. Cannot account for cannabis potency, frequency, or trimester of use. Relatively small sample size.