A large cohort study from Canada has found that around 2% of pregnant women used cannabis during the study period and that this use was particularly associated with a greater infant risk of preterm birth, low birth weight and small-for-gestational age.
The use of cannabis in pregnancy ‘increases the likelihood’ of preterm birth, low birth weight, small-for-gestational age and major congenital anomalies in prenatally exposed female infants, according to findings from a cohort study comprising more than 1.2 million singleton pregnancies recorded between 2012 and 2019.(1) The study was based on data from three provinces in Canada (Ontario, British Columbia, Newfoundland), said to represent nearly half of all Canadian pregnancies.
With relaxed legislation in some US states and Canadian provinces, culminating in the latter with the Cannabis Act of 2018, legalising ‘the possession, distribution, sale and production of cannabis in Canada’, Canada has become a fertile ground for studies on the effects of cannabis. However, because cannabinoids, like tobacco, readily cross the placenta, an understanding of their effect in pregnancy is urgently needed.
With prenatal records in the three provinces disclosing self-reported drug use (as well as tobacco and alcohol), the outcomes of interest for this study included low birth weight, small-for-gestational age, large-for-gestational age, spontaneous and medically indicated preterm birth, very preterm birth, stillbirth, major congenital anomalies, caesarean section, gestational diabetes and gestational hypertension. All births were singleton, to exclude confounding from multiples, and this left an eligible cohort of 1,280,447 live and stillbirth pregnancies. Models for maternal and infant outcomes were adjusted for non-cannabis substance use, maternal age, BMI, parity, pre-existing and gestational diabetes, and chronic and gestational hypertension.
Results first showed that overall prevalence of cannabis use throughout the study period was 2%, but this rate increased over time from 1.5% in 2012/2013 to 2.5% in 2018/2019. Pregnant individuals who used cannabis were more frequently aged 24 or younger (p<0.0001). There were also strong associations with infant and maternal outcomes, notably in low birth weight, preterm birth and small-for-gestational age. More specifically, adjusted regression models indicated that pregnant individuals who used cannabis had 85% higher odds of preterm birth (OR 1.85), very preterm birth (OR 1.73) and low birth weight (OR 1.90). There were also 21% higher odds of an infant being small-for-gestational age and 71% higher odds of any major congenital anomaly. No association was found for stillbirth. For maternal outcomes, prenatal cannabis use was associated with increased risk of gestational diabetes (OR 1.32) and caesarean section (OR 1.13) but there was no statistically significant association with gestational hypertension.
When does association imply causation? Can cannabis use explain these adverse outcomes? First, there must be biological plausibility, and the authors cite a 2020 study showing that the major psychoactive ingredient in cannabis (Δ9-THC) causes fetal growth restriction, albeit through mechanisms which ‘are not well understood’. They also report previous findings that cannabis use increases plasma cortisol levels in pregnant individuals, with higher levels of cortisol known to increase the risk for preterm birth. The latter was the main outcome measure of a similar cohort study (also in Canada), which found a ‘significant’ association of cannabis use in pregnancy with preterm birth.(2) The most commonly attributed biological finding in all studies, however, is simply that cannabis, like tobacco, crosses the placenta and enters the fetal bloodstream.
This earlier Canadian study also found a similar overall prevalence rate of 2% usage among pregnant women – and even higher in those under 24 years. Although the latest study gives no indication of an absolute background risk for the adverse pregnancy outcomes, the prevalence rate is deemed sufficiently high to warrant a warning from the authors that ‘additional measures are needed to inform the public and providers of the inherent risks of cannabis exposure in pregnancy’, and adding: ‘Greater efforts are required to inform the public, to communicate evidence-based information during prenatal visits and to support shared decision making.’
1. Luke S, Hobbs AJ, Smith M, et al. Cannabis use in pregnancy and maternal and infant outcomes: A Canadian cross-jurisdictional population-based cohort study. PLoS ONE 2022; 17(11): e0276824. https://doi.org/10.1371/journal
2. Corsi DJ, Walsh L, Weiss D, et al. Association between self-reported prenatal cannabis use and maternal, perinatal, and neonatal outcomes. JAMA 2019; 322: 145–152. doi.org/10.1001/jama.2019.8734