Cannabis increases risk of unhealthy pregnancy outcomes

In the past 10 years, the percentage of Americans who use medical marijuana has more than doubled as state-level legalisation becomes increasingly common. But the full health effects of cannabis remain unknown, especially for specific populations — such as pregnant people — that might be at risk of health complications.

Researchers at the University of Utah Health were driven to address this issue in part by the contradictory answers that many people encounter when trying to learn about the health impacts of cannabis use. Indeed, some previous studies on the topic found no association between cannabis use and pregnancy complications, with one hurdle being the fact that there are so many differences between the baseline characteristics of people who use and don’t use cannabis during pregnancy.

Now, a study of more than 9000 pregnant people from eight medical centres across the US has allowed the researchers to statistically untangle the impacts of cannabis use from many other factors, including pre-existing health conditions, nicotine exposure and socioeconomic status. Furthermore, while other studies have asked participants to report their own cannabis use, the scientists measured the levels of a metabolic by-product of cannabis in participants’ urine samples, which gave more accurate measurements of cannabis exposure.

The scientists found that cannabis exposure during pregnancy is associated with a 1.3-fold increase in risk of unhealthy pregnancy outcomes after the impacts of other factors were removed. Higher levels of cannabis exposure over the course of pregnancy were associated with higher risks. The results were published in JAMA.

To gauge impacts on pregnancy, the researchers looked at an aggregate measure of negative health outcomes, including low birth weight, pregnancy-related high blood pressure, stillbirth and medically indicated preterm birth. Of these, the association between cannabis use and low birth weight was the strongest. All of these conditions have been linked to reduced function of the placenta, which supplies the growing baby with oxygen and nutrients. And while this type of study cannot determine why cannabis is associated with negative pregnancy outcomes, previous studies in non-human primates have found that long-term cannabis exposure can interfere with blood supply to the placenta, so it is hypothesised that cannabis may disrupt the human placenta in a similar way.

“Cannabis use is not safe,” said Dr Robert Silver, last author on the study. “It increases the risk of pregnancy complications. If possible, you shouldn’t use cannabis during pregnancy.”

Silver added that the greater risk seen at higher levels of exposure is especially concerning given the high amount of THC found in newer cannabis products — products that were barely starting to become available from 2010 to 2014, when the study data was collected. The health impacts of these more concentrated products remain largely unknown.

The researchers urged people who are considering using cannabis while pregnant to have an open conversation with their doctor. While pregnant people may turn to cannabis to alleviate nausea or anxiety, other remedies — such as therapy, exercise, meditation and SSRIs — have been proven to be safe and effective. Meanwhile, said Silver, continued research on the health impacts of cannabis is urgently needed so that patients can make informed decisions about their health.

Image caption: Torri Metz, lead author on the study, takes an ultrasound of a pregnant person’s belly.