High-potency prenatal cannabis exposure linked to lower birth weight and shorter length in low-risk pregnancies | Research

The takeaway

In a carefully controlled study excluding other substances, newborns exposed to high-potency cannabis (averaging 198 mg THC/day) weighed less and were shorter, with female newborns showing reduced head circumference.

Obstetricians, maternal-fetal medicine specialists, and prenatal cannabis researchers

198 mg THC/day average exposure; birth weight 38th vs 52nd percentile

What the researchers found

Among 37 cannabis-exposed and 35 control newborns in otherwise low-risk pregnancies, cannabis-exposed newborns weighed less (38th vs 52nd percentile, p = 0.04) and were shorter (40th vs 55th percentile, p = 0.03). Female exposed newborns had significantly smaller head circumference (28th percentile) compared to males (55th percentile, p = 0.02).

Why it matters

This is one of the few studies to isolate cannabis effects from other substances in pregnancy. The high-potency exposure (average 198 mg THC/day) and sex-specific head circumference findings are particularly notable.

The numbers in context

37 exposed vs 35 controls; average 198 mg THC/day and 3.5 mg CBD/day; birth weight: 38th vs 52nd percentile (p = 0.04); length: 40th vs 55th percentile (p = 0.03); female head circumference 28th percentile vs male 55th percentile (p = 0.02)

How the study worked

Prospective observational cohort study (2019-2022) in Washington and Oregon. Cannabis-exposed women used cannabis at least 3 days/week during the first trimester. All participants were screened for no alcohol, tobacco, or illicit drug use via urine toxicology at multiple time points. Cannabis use was quantified using product weight and potency.

What this study cannot tell us

Small sample size; no randomization; self-selected groups; cannabis use quantified but not biomarker-verified; cannot rule out unmeasured lifestyle confounders; short-term birth outcomes only

How to read the evidence

Prospective cohort with substance use monitoring via toxicology screens, but small sample and non-randomized design.

When this study was published

2024 study with 2019-2022 recruitment

The bigger picture

The sex-specific effect on head circumference raises questions about whether high-potency cannabis differentially affects female brain development in utero, which has implications for long-term neurodevelopmental outcomes.

Questions still open

Why would female fetuses be more susceptible to head circumference reduction from cannabis exposure? Do these birth outcome differences translate into developmental differences later? Is there a dose threshold below which effects are not observed?

Common questions

How much cannabis were pregnant women using?

The cannabis-exposed group averaged 198 mg of THC per day (about 3.5 mg CBD/day), which is considered high-potency use. Women were required to use cannabis at least 3 days per week during the first trimester to qualify for the study.

Were the effects different for boys and girls?

Yes. While both sexes showed lower birth weight and shorter length, female newborns exposed to cannabis had significantly smaller head circumference (28th percentile) compared to exposed males (55th percentile). This sex difference suggests cannabis may affect brain growth differently depending on fetal sex.

Read the original research

High-Potency Prenatal Cannabis Exposure and Birth Outcome Measures.

Children (Basel, Switzerland), 11(12)

View on PubMed ↗Publisher’s paper ↗

Citation

Kleinhans, Natalia M; Johnson, Allegra J; Larsen, Sarah F; Berkelhamer, Sara K; Larimer, Mary E; Dager, Stephen R. (2024). High-Potency Prenatal Cannabis Exposure and Birth Outcome Measures.. Children (Basel, Switzerland), 11(12). https://doi.org/10.3390/children11121436