Among 1,540 newborns with meconium drug testing, those positive for THC had significantly lower birth weight (0.16 kg less) and nearly twice the odds of low birth weight compared to unexposed infants.
Pregnant individuals who use cannabis, obstetricians, and neonatologists.
1.9x odds of low birth weight with prenatal THC exposure
What the researchers found
Neonates exposed to THC had significantly lower birth weight, head circumference, and length (all p<0.001). THC-exposed neonates had 1.9 times the odds of low birth weight (95% CI 1.3-2.7). Birth weight was on average 0.16 kg lower in THC-exposed infants after controlling for confounders.
Why it matters
This study overcomes a major limitation of prior research by using meconium drug testing instead of maternal self-report, providing more reliable exposure data. The consistent findings across multiple growth measures strengthen the evidence.
The numbers in context
1,540 meconium screens analyzed. 483 positive for THC only. Birth weight 0.16 kg lower (95% CI 0.10-0.22, p<0.001). OR for low birth weight: 1.9 (95% CI 1.3-2.7). Significantly lower head circumference and length as well.
How the study worked
Retrospective cohort using meconium drug screens from infants born in a Pacific Northwest hospital system over 2.5 years. 1,540 screens analyzed: 483 positive for THC only (no other drugs) compared to negative screens. Multivariable regression controlled for confounders.
What this study cannot tell us
Retrospective design. Meconium testing confirms exposure but does not measure dose, timing, or frequency. Hospital system population may not generalize. Unknown confounders despite multivariable adjustment. Single geographic region.
How to read the evidence
Large retrospective cohort using objective exposure measurement (meconium testing) with multivariable adjustment.
When this study was published
Published in 2022.
The bigger picture
By isolating THC-only exposure (excluding polysubstance use) and using objective meconium testing, this study provides some of the cleanest evidence linking prenatal cannabis exposure to reduced fetal growth.
Questions still open
- Is there a dose-response relationship between prenatal THC exposure and birth weight reduction? Do these growth differences persist into childhood?
Common questions
Does using marijuana during pregnancy affect birth weight?
How was marijuana exposure confirmed in this study?
Read the original research
Prenatal marijuana exposure and neonatal outcomes: a retrospective cohort study.
BMJ open, 12(9), e061167
Citation
Jones, Michael James; Lotfi, Asma; Lin, Amber; Gievers, Ladawna L; Hendrickson, Robert; Sheridan, David C. (2022). Prenatal marijuana exposure and neonatal outcomes: a retrospective cohort study.. BMJ open, 12(9), e061167. https://doi.org/10.1136/bmjopen-2022-061167
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