A large meta-analysis of over 19 million subjects found prenatal cannabis exposure associated with a 25-33% increased risk of birth defects, with significant associations for cardiovascular, GI, nervous system, genitourinary, and musculoskeletal defects.
Pregnant individuals, obstetricians, epidemiologists interested in meta-analysis methodology
ORs 1.25-1.33 for any birth defect
What the researchers found
Pooling data from 18 cohort and 18 case-control studies (>19 million subjects), prenatal cannabis exposure was associated with any birth defect (pooled ORs 1.25-1.33). ORs were also elevated for cardiovascular, gastrointestinal, nervous system, genitourinary, and musculoskeletal defects but not orofacial defects. Effects were smaller and less often significant in adjusted analyses.
Why it matters
5-10% of US pregnancies are exposed to cannabis, with highest use in the first trimester when organs are forming. Even a small increase in birth defect risk could affect a meaningful number of pregnancies at the population level.
The numbers in context
36 studies (18 cohort, 18 case-control), >19 million subjects. Pooled ORs for any birth defect: 1.25-1.33. Significant for cardiovascular, GI, nervous system, genitourinary, and musculoskeletal defects. Not significant for orofacial defects. Adjusted estimates were smaller.
How the study worked
Review and critical appraisal of two meta-analyses pooling cohort and case-control studies of prenatal cannabis exposure and birth defects. Discusses both unadjusted and adjusted estimates, cumulative meta-analyses, and methodological strengths and shortcomings.
What this study cannot tell us
The meta-analysis pooled cohort and case-control ORs together, which is methodologically problematic. Some forest plots included the same sample multiple times. Adjusted analyses showed weaker associations. First-trimester exposure was not always distinguishable from overall pregnancy exposure.
How to read the evidence
Large meta-analysis with >19 million subjects, but methodological issues (mixed study designs, repeated samples) and weaker adjusted estimates limit certainty.
When this study was published
2024 review of recent meta-analyses on cannabis and birth defects
The bigger picture
This analysis also serves as a methodological teaching tool, highlighting how pooling different study designs and repeated representation of samples in forest plots can affect meta-analysis results. The cumulative meta-analysis showed when findings became consistently significant over time.
Questions still open
- Are the birth defect associations driven by cannabis itself or by confounding factors like tobacco co-use? Does timing of exposure (first trimester vs. later) matter for specific defect types?
Common questions
How big is the increased risk?
Which birth defects were most associated with cannabis?
Read the original research
Towards a Further Understanding of Meta-Analysis Using Gestational Exposure to Cannabis and Birth Defects as a Case in Point.
The Journal of clinical psychiatry, 85(4)
Citation
Andrade, Chittaranjan. (2024). Towards a Further Understanding of Meta-Analysis Using Gestational Exposure to Cannabis and Birth Defects as a Case in Point.. The Journal of clinical psychiatry, 85(4). https://doi.org/10.4088/JCP.24f15673
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