Using marijuana during pregnancy has been linked to poor foetal development, low infant birth weight, dangerously early deliveries and even death, according to a recent large meta-analysis of research.
“The most striking finding is the increased risk of perinatal mortality – death either during the pregnancy or shortly after the pregnancy,” said obstetrician and lead study author Dr Jamie Lo, an associate professor of obstetrics, gynaecology and urology in the School of Medicine at Oregon Health & Science University in Portland.
These medical outcomes are known despite the lack of gold-standard clinical trials asking pregnant women to drink alcohol, smoke tobacco, take heroin, cocaine, oxycodone or fentanyl and then compare the outcomes to pregnant women who abstained. Such research, of course, would be grossly unethical, reports CNN.
Thus, because a randomised clinical trial of the impact of cannabis on a foetus will never occur, scientists focus on the outcome of self-reported use by pregnant mothers.
“This systematic review is unique in that we only reviewed studies in which cannabis was used during pregnancy,” Lo said. “Prior work has included studies which also looked at cannabis use along with other substances like nicotine or alcohol.”
Quality of evidence rises as more studies done
The latest research, published in JAMA Paediatrics, analysed 51 studies with more than 21m participants.
Use of marijuana during pregnancy was linked to a 52% higher risk of preterm delivery before 37 weeks, and a 75% higher risk of low birth weight, which is less than 2 500 grams at delivery.
Only six studies looked at the impact of cannabis on mortality – and found a 29% higher risk of infant death associated with marijuana use in pregnancy.
The new meta-analysis used the GRADE (grading of recommendations assessment, development, and evaluation) approach to rating the quality of each study. In a previous analysis published in 2024, Lo and her team rated available studies as very low or low certainty, meaning the evidence in the studies was limited and the findings not reliable.
Just a year later, existing evidence was upgraded to low to moderate certainty. A moderate grade indicates researchers are reasonably confident in using that information for decision-making but recognise that future research might refine conclusions or recommendations.
“Research is evolving quickly in this area,” said Brianna Moore, assistant professor of epidemiology at the Colorado School of Public Health Anschutz Medical Campus in Aurora. She was not involved in the new study.
“This review found that as more studies are conducted with consistent results, there is more certainty of an association between prenatal exposure to cannabis and adverse birth outcomes.”
‘No clear public health messaging’
In addition to believing that marijuana is safe because it’s natural, expectant parents are also getting mixed messages about the health harms of cannabis.
“Perceptions of safety are compounded by the increased availability and legalisation of cannabis,” Lo said. “In addition, healthcare providers are poor at counselling due to confusion over conflicting studies. Therefore, there’s no clear public health messaging.”
Historically it has been difficult to investigate cannabis use because weed has been illegal – and still is – in many US states, while any studies that were undertaken had to follow strict federal regulations.
Modern research into the potential harms of cannabis is still in its infancy. Older studies, often done in the 1980s when marijuana was much less potent, may not reflect today’s reality, experts say.
Research over the past decade has linked marijuana use to cognitive decline and dementia, complications during elective surgery, and an increased risk of some cancers. Users are nearly 25% more likely to need emergency care and hospitalisation, according to a 2022 study.
Any level of marijuana use may raise the risk of stroke by 42% and heart attack by 25%, even if there is no prior history of heart disease and the person has never smoked or vaped tobacco.
It has also been linked to cardiac arrhythmias such as atrial fibrillation; myocarditis – an inflammation of the heart muscle – spasms of the heart’s arteries and a higher risk of heart failure.
Young people who use marijuana are more likely to develop long-lasting mental disorders, including depression, social anxiety and schizophrenia, and drop out of school, the CDC said. Studies show overuse by youth with mood disorders leads to a rise in self-harm, suicide attempts and death.
Daily use by adolescents and adults can result in another unpleasant side effect: uncontrollable vomiting, according to a 2021 study. And a 2020 study found children born to marijuana users had more psychotic-like behaviours and more attention, social and sleep problems, as well as weaker cognitive abilities.
“Ideally, it’s best not to be exposed to THC, which is the psychoactive ingredient of cannabis, no matter what form you’re using,” Lo said.
Study details
Prenatal Cannabis Use and Neonatal Outcomes: A Systematic Review and Meta-Analysis
Jamie Lo, Chelsea Ayers, Snehapriya Yeddala, et al.
Published in JAMA Paediatrics on 5 May 2025
Abstract
Importance
Prenatal cannabis use continues to increase, and cannabis remains the most commonly used illegal substance in pregnancy. Accumulating evidence suggests potential adverse effects on foetal and neonatal outcomes following cannabis use in pregnancy.
Objective
To update a living systematic review and meta-analysis to provide a timely understanding regarding cannabis use in pregnancy and foetal and neonatal outcomes.
Data Sources
The previous review was updated by searching bibliographic databases MEDLINE, CINAHL, PsycInfo, Global Health, and Evidence-Based Medicine Reviews Cochrane Database of Systematic Reviews from November 1, 2021, to April 4, 2024.
Study Selection
Cohort or case-control studies comparing pregnancies with and without prenatal cannabis use on pre-specified foetal or neonatal outcomes with adjustment for confounders, such as co-use of tobacco products, were included. Two independent reviewers screened studies, with disagreements resolved through discussion.
Data Extraction and Synthesis
Included studies were extracted by 1 reviewer and confirmed by a second. Risk of bias was assessed with the Newcastle-Ottawa Scale. Random-effects meta-analyses of unadjusted and adjusted odds ratios (ORs) were performed for all primary outcomes. Results were synthesised using the Grading of Recommendations Assessment, Development, and Evaluation approach.
Main Outcomes and Measures
Primary outcomes were preterm birth (PTB; <37 weeks of gestation), small for gestational age (SGA), low birth weight (LBW; <2500 g), and perinatal mortality.
Results
For this update, 8 new studies with 1 709 998 participants were added, for a total of 51 studies synthesized (N = 21 146 938). From meta-analyses of adjusted effect sizes, moderate-certainty evidence indicated that cannabis use in pregnancy was associated with increased odds of LBW (20 studies; OR, 1.75; 95% CI, 1.41-2.18), PTB (20 studies; OR, 1.52; 95% CI, 1.26-1.83), and SGA (12 studies; OR, 1.57; 95% CI, 1.36-1.81), and low-certainty evidence indicated that it was associated with greater odds of perinatal mortality (6 studies; OR, 1.29; 95% CI, 1.07-1.55). Previously, the evidence was rated as very low or low certainty.
Conclusions and Relevance
Cannabis use in pregnancy was associated with greater odds of PTB, SGA, and LBW even after adjusting for co-use of tobacco products, and confidence in these findings increased from low in the prior review to moderate in the current meta-analysis. The findings of this study may help inform patient counselling and future public health policies.
CNN article – Marijuana is extremely dangerous to the foetus in the womb, study finds (Open access)
See more from MedicalBrief archives:
Foetal growth stunted by cannabis in pregnancy – US study
Cannabis use in pregnancy boosts risk of child sleep problems
Prenatal cannabis exposure linked to adolescent mental health issues