Does cannabis use increase a woman’s risk for PTB?

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Because cannabis has antiemetic effects and legalization is making it increasingly available in some states, many pregnant patients have turned to the drug for relief of hyperemesis. But little is known about adverse maternal, perinatal, and neonatal outcomes associated with it, a research gap addressed in a population-based retrospective cohort study that appears in JAMA. 

The study included singleton live births and stillbirths among women 15 years or older in Ontario, Canada between April 2012 and December 2017, using data from Ontario’s Better Outcomes Registry & Network (BORN). 

The primary outcome of the study was preterm birth (PTB) before 37 weeks’ gestation. Ten secondary outcomes were also examined, including small for gestational age (SGA), placental abruption, transfer to neonatal intensive care unit (NICU), and 5-minute Apgar score. 

The study cohort included 661,617 women with a mean age of 30.4 years. Of them, 9,427 (1.4%) reported using cannabis during their pregnancy. Their mean gestational age was 39.3 weeks and 51% of the infants were male. Through sociodemographic matching, the researchers identified a matched cohort of 5,639 reported users and 92,873 reported nonusers.

For cannabis users, the crude rate of PTB was 12.0% versus 6.1% among women who did not report using cannabis in the unmatched cohort (risk differences [RD], 5.88% [95% CI 5.22% - 6.54%). Reported cannabis use in the unmatched cohort was also associated with significant increases in SGA (3^rdpercentile and 10^thpercentile), placental abruption, admission to the NICU, and a 5-minute Apgar score less than 4 in the unmatched cohort. Among women who reported use of cannabis but no other substance, the crude rate of PTB was 9.1% compared with 5.9% in women who reported no substance use.

In the matched cohort, reported cannabis exposure was significantly associated with an RD of 2.98% (95% CI, 2.63% - 3.34%) and a relative risk (RR) of 1.41 (95% CI 1.36-1.47) for PTB at less than 37 weeks’ gestation. Cannabis exposure also had a statistically significant protective association with preeclampsia (RR 0.90, 95% CI 0.86-0.95) as well as gestational diabetes (RR 0.91, 95% CI 0.86-0.96). On the other hand, cannabis exposure was inversely associated with cesarean vs spontaneous vaginal delivery (RR 0.98, 95% CI 0.96-1.00), although the RD was not significant (RD –0.33%, 95% CI –0.85-0.18).

This study supports previous findings of the significant associations between cannabis exposure and increased risk for PTB. However, the authors believe the results of their study are important because prior studies experienced a high likelihood of residual confounding due to misclassification of cannabis exposure and other confounders such as tobacco or other substance abuse. As more states and countries legalize cannabis, it is expected that use of the drug may increase, so ob/gyns should be prepared to discuss potential risks with their pregnant patients.