An updated systematic review suggests cannabis use in pregnancy raises the risk of serious outcomes, including preterm birth, low birth weight, and infant death.
A newly published study in JAMA Pediatrics by researchers at Oregon Health & Science University (OHSU) strengthens the evidence that cannabis use during pregnancy may pose significant health risks to developing babies.
Increased Use Despite Perceived Safety
Jamie Lo, M.D., M.C.R., the study’s lead author and an associate professor of obstetrics and gynecology (maternal-fetal medicine) at OHSU, said more of her prenatal patients are asking whether cannabis is safe to use during pregnancy.
“Patients are coming to me in their prenatal visits saying, ‘I quit smoking and drinking, but is it safe to still use cannabis?’ Until direct harms have been proven, they perceive it to be safe to use,” said Dr. Lo.
Despite federal classification of cannabis as a Schedule 1 controlled substance, use during pregnancy has been rising—unlike the declining trends seen with alcohol and nicotine. Many pregnant individuals report using cannabis to ease nausea, insomnia, and pain.
Large Review Strengthens the Link
This updated systematic review and meta-analysis evaluated 51 observational studies involving more than 21 million individuals. Compared to previous reviews, this one included eight new studies and more cases where cannabis was used in isolation (without nicotine), strengthening the reliability of the findings.
The analysis showed:
Moderate-certainty evidence links cannabis use during pregnancy to increased odds of:
Low birth weight
Preterm birth
Small size for gestational age
Low-certainty evidence also suggests a higher risk of newborn mortality.
Animal Models Support Human Findings
Animal studies using primate models exposed to THC—the main psychoactive compound in cannabis—have shown that cannabis negatively affects the placenta. Imaging revealed:
- Reduced blood flow
- Lower oxygen availability
- Decreased volume of amniotic fluid
“These findings tell me as an obstetrician that the placenta is not functioning as it normally would in pregnancy,” Dr. Lo said. “When the placenta isn’t functioning well, it can affect the baby’s development and growth.”
A Realistic Approach: Harm Reduction
In Oregon and other states where cannabis is legal for medicinal and recreational use, total abstinence may not always feel achievable for patients. Dr. Lo emphasizes a harm-reduction approach.
“Even using less can mitigate the risk,” she said. “Abstinence is ideal, but it’s not realistic for many patients.”
Instead of judging or dismissing patients who use cannabis, Dr. Lo and her colleagues encourage open, honest discussions to reduce harm.
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