Researchers from The Ottawa Hospital, CHEO, BORN Ontario and the University of Ottawa have conducted one of the largest studies to date on the effect of cannabis use during pregnancy. They found that reported cannabis use during pregnancy was associated with an increased risk of preterm birth. The findings were published in JAMA.

“It’s worth warning people of the risks of consuming cannabis during pregnancy,” said Dr. Mark Walker, Chief of the Department of Obstetrics, Gynecology and Newborn Care at The Ottawa Hospital, professor at the University of Ottawa and senior author on the study. “We hope our findings will help women and their healthcare providers make informed decisions.”

Babies born preterm, before 37 weeks gestation, may have complications and need more care in the first few weeks of life. The later the baby is born, the milder the symptoms. Most of the preterm births in the study were moderate to late preterm, born between 32 and 36 weeks gestation.

The research team reviewed birth registry data from BORN Ontario collected between 2012 and 2017, prior to the legalization of recreational cannabis in Canada. BORN Ontario, delivered by CHEO, collects data on pregnancy, birth and their outcomes for every birth in Ontario. Of the 661,617 women in the study, 9,427 (1.4 percent) reported using cannabis during pregnancy.

Dr. Mark WalkerThe researchers found that the rate of preterm birth among reported cannabis users was 12 percent compared to 6 percent in non-users.

However, the women who reported cannabis use often had other risk factors: 59 percent smoked tobacco, 19 percent consumed alcohol and 11 percent used opioids during pregnancy. To try to tease out which health effects were only attributable to cannabis, the researchers matched these individuals to people with similar risk factors who did not use cannabis.

Analysis of these matched groups showed a 10.2 percent risk of preterm birth among reported cannabis users compared to a 7.2 percent risk in non-users with similar risk factors. Reported users also had 19.3 percent risk of transfer to a neonatal intensive care unit, compared to 13.8 percent for non-users with similar risk factors.

“In the past we haven’t had good data on the effect of cannabis on pregnancies,” said Dr. Daniel Corsi, Epidemiologist at The Ottawa Hospital and BORN Ontario. “This is one of the largest studies on this topic to date.”

The researchers do not know how much cannabis women were using, how often, at what time during their pregnancy, or how it was consumed. They also note that while they tried to control for other factors that could influence birth outcomes, their study can still only show association – not cause and effect.  

Little is known about the health impacts of cannabis use during pregnancy. Health Canada and the Society of Obstetricians and Gynaecologists of Canada recommend that it is safest to avoid using cannabis when pregnant and breastfeeding until more is known about the short and long-term effects.

Women who wish to stop using cannabis during their pregnancy should ask their health care provider about support and services in their region. Dr. Daniel Corsi

“We know from previous studies that many women think using cannabis during pregnancy is a low-risk activity,” said Dr. Walker. “As health care providers, it’s our responsibility to ask pregnant women about their cannabis use and to make sure they know the facts.”

Reference: “Association Between Self-reported Prenatal Cannabis Use and Maternal, Perinatal, and Neonatal Outcomes”. Daniel J. Corsi, Laura Walsh, Deborah Weiss, Helen Hsu, Darine El-Chaar, Steven Hawken, Deshayne B. Fell, Mark Walker. JAMA. June 18, 2109. doi:10.1001/jama.2019.8734

Funders: This study was funded by the Canadian Institutes of Health Research. Research at The Ottawa Hospital is possible thanks to generous donations to The Ottawa Hospital Foundation. BORN Ontario is supported by the Ontario Ministry of Health and Long Term Care.

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