How Has Cannabis Legalization Affected Pregnant Mothers?

After Canada legalized recreational cannabis in 2018, the rate of acute care for cannabis use during pregnancy in Ontario nearly doubled, data indicate.

A population-based study shows that the rate of cannabis-related acute care use during pregnancy increased from 11 per 100,000 pregnancies before legalization to 20 per 100,000 pregnancies afterwards: an increase of 82%. Absolute increases were small, however.



Dr Daniel Myran

“Our findings are consistent with studies highlighting that cannabis use during pregnancy has been increasing in North America, and this study suggests that cannabis legalization might contribute to and accelerate such trends,” study author Daniel Myran, MD, MPH, a public health and preventive medicine physician at the University of Ottawa in Ontario, Canada, told Medscape Medical News.

The study was published online May 23 in the Canadian Medical Association Journal.

Risks for Newborns  

In a 2019 study, 7% of US women reported using cannabis during pregnancy in 2016-2017, which was double the rate of 3.4% in 2002-2003.

Myran and colleagues hypothesized that legalizing nonmedical cannabis has affected the drug’s use during pregnancy in Ontario. “We also hypothesized that hospital care for cannabis use would be associated with adverse neonatal outcomes, even after adjusting for other important risk factors that may differ between people with and without cannabis use,” he said.

The researchers’ repeated cross-sectional analysis evaluated changes in the number of pregnant people who received acute care from January 2015 to July 2021 among all patients who were eligible for Ontario’s public health coverage. The final study cohort included 691,242 pregnant patients, of whom 533 had at least one pregnancy with cannabis-related acute care visits. These mothers had a mean age of 24 years vs 30 for their counterparts with no such visits.

Using segmented regression, the researchers compared changes in the quarterly rate of pregnant people with acute care related to cannabis use (the primary outcome) with those of acute care for mental health conditions or for noncannabis substance use (the control conditions).

“Severe morning sickness was a major risk factor for care in the emergency department or hospital for cannabis use,” said Myran. “Prior work has found that people who use cannabis during pregnancy often state that it was used to manage challenging symptoms of pregnancy such as morning sickness.”

Most acute care events (72.2%) were emergency department visits. The most common reasons for acute care were harmful cannabis use (57.6%), followed by cannabis dependence or withdrawal (21.5%), and acute cannabis intoxication (12.8%).

Compared with pregnancies without acute care, those with acute care related to cannabis had higher rates of adverse neonatal outcomes such as birth before 37 weeks’ gestational age (16.9% vs 7.2%), birth weight at or below the bottom fifth percentile after adjustment for gestational age (12.1% vs 4.4%), and neonatal intensive care unit admission in the first 28 days of life (31.5% vs 13%).

An adjusted analysis found that patients younger than 35 years and those living in rural settings or the lowest-income neighborhoods had higher odds of acute cannabis-related care during pregnancy. Patients who received acute care for any substance use or schizophrenia before pregnancy or who accessed outpatient mental health services before pregnancy had higher risk for cannabis-related acute care during pregnancy. Mothers receiving acute care for cannabis also had higher risk for acute care for hyperemesis gravidarum during pregnancy (30.9%). 

The rate of acute care for other types of substance use such as alcohol and opioids did not change after cannabis legalization, and acute care for mental health conditions such as anxiety and depression during pregnancy declined by 14%, Myran noted.

“Physicians who care for pregnant people should consider increasing screening for cannabis use during pregnancy,” said Myran. “In addition, repeated nonstigmatizing screening and counseling may be indicated for higher-risk groups identified in the study, including pregnancies with severe morning sickness.”

The US Perspective

Commenting on the study for Medscape, M. Camille Hoffman, MD, MSc, a maternal-fetal medicine specialist at the University of Colorado School of Medicine in Aurora, said that the findings likely indicate that legalization has made cannabis users less reluctant to come forward for urgent care. “They cannot really claim that this is equivalent to more use, just that more people are willing to present,” she said. Hoffman was not involved in the study.



Dr M. Camille Hoffman

The Canadian results do not align perfectly with what is seen in the United States. “It does suggest that there may be more cannabinoid hyperemesis being coded as hyperemesis gravidarum, which is a pregnancy-specific condition vs a cannabis-dependence-related one,” said Hoffman.

Literature in the US often includes tobacco use as a covariate, she added. “This study does not appear to do that,” she said. “Rather, it uses any substance use. Because of this, it is difficult to really know the contribution of cannabis to the adverse pregnancy outcomes vs the combination of tobacco and cannabis.”

Finally, she pointed out, the proportion of those presenting for acute care for substance use in the 2 years before conception was 22% for acute care visits for cannabis vs 1% for no acute care visits. “This suggests to me that this was a highly vulnerable group before the legalization of cannabis as well. The overall absolute difference is nine in total per 100,000 — hardly enough to draw any real conclusions. Again, maybe those nine were simply more willing to come forth with concerns with cannabis being legal.” 

There is no known safe level of cannabis consumption, and its use by pregnant women has been linked to later neurodevelopmental issues in their offspring. A 2022 US study suggested that cannabis exposure in the womb may leave children later in life at risk for autism, psychiatric disorders, and problematic substance abuse, particularly as they enter peak periods of vulnerability in late adolescence.

As to the impact of legalization in certain US states, a 2019 study found that women perceived legalization to mean greater access to cannabis, increased acceptance of use, and greater trust in cannabis retailers. In line with Hoffman’s view, this study suggested that legalization made pregnant women more willing to discuss cannabis use during pregnancy honestly with their care providers.

In the US, prenatal cannabis use is still included in definitions of child abuse or neglect and can lead to termination of parental rights, even in states with full legalization.

“These findings highlight the need for ongoing monitoring of markers of cannabis use during pregnancy after legalization,” said Myran. He also called for effective policies in regions with legal cannabis, such as increased warning labels on cannabis products.

This study was supported by the Canadian Institutes of Health Research and the University of Ottawa site of ICES, which is funded by an annual grant from the Ontario Ministry of Health and Ministry of Long-Term Care. Myran reports a speaker fee from McMaster University. Hoffman reports no relevant financial relationships.

CMAJ. Published online May 23, 2023. Full text.

Diana Swift is a medical journalist based in Toronto, Canada.

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