Babies exposed to cannabis in utero showed significant reductions in head circumference detectable as early as the second trimester, with a deficit of more than 14 percentile points by birth, even after controlling for other substance exposure.
Pregnant individuals, prenatal care providers, anyone considering cannabis use during pregnancy.
14+ percentile point head circumference deficit by birth in cannabis-exposed pregnancies
What the researchers found
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.
Why it matters
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.
The numbers in context
109 cannabis-exposed vs 171 controls. Head circumference deficit: more than 14 percentile points by birth. Head size differences significant starting at second-trimester ultrasound. Birth weight significantly reduced. Length not significantly affected.
How the study worked
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.
What this study cannot tell us
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.
How to read the evidence
Well-controlled cohort excluding other substance users, but limited by small sample size and observational design.
When this study was published
Published 2023, using 2010-2020 clinical data.
The bigger picture
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.
Questions still open
- Do the head circumference deficits translate to measurable cognitive or developmental differences in childhood?
- Does the timing or amount of cannabis use during pregnancy affect the degree of growth restriction?
Common questions
Does cannabis use during pregnancy affect fetal growth?
Were other substances involved?
Read the original research
The impact of in utero cannabis exposure on fetal growth.
Journal of neonatal-perinatal medicine, 16(3), 485-490
Citation
Nadolski, K; Dodge, P; Kopkau, H; Forrestal, K; Zablocki, V; Bailey, B A. (2023). The impact of in utero cannabis exposure on fetal growth.. Journal of neonatal-perinatal medicine, 16(3), 485-490. https://doi.org/10.3233/NPM-221133
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