In the past ten years, the percentage of Americans who use medical marijuana has more than doubled as state-level legalization becomes increasingly common. But despite its prevalence as a medication, the full health effects of cannabis remain unknown, especially for specific populations—such as pregnant people—that might be especially at risk of health complications.
Now, in a large study of more than 9,000 pregnant people from across the U.S., researchers at University of Utah Health have found that cannabis exposure during pregnancy is associated with a composite measure of unhealthy pregnancy outcomes, especially low birth weight, and that higher exposure is associated with higher risks.
Compared to most prior studies, this study was larger and measured cannabis exposure more accurately, which allowed researchers to distinguish the effects of cannabis itself from those caused by other correlated health conditions. The research published online on December 12 in JAMA.
The large study population, including participants from eight medical centers across the U.S., allowed the researchers to address this issue. Being able to compare pregnancy outcomes for so many participants, 610 of which had detectable levels of cannabis exposure, meant that the researchers could statistically untangle the impacts of cannabis use from many other factors, including pre-existing health conditions, nicotine exposure, and socioeconomic status.
The scientists found that cannabis exposure was associated with a 1.3-fold increase in risk after the impacts of other factors were removed. Higher levels of cannabis exposure over the course of pregnancy were associated with higher risks.
A distinguishing feature of the study was how the researchers measured cannabis exposure. While other studies had asked participants to report their own cannabis use (which has been shown to underestimate the actual rate of use by two or three times), the scientists measured the levels of a metabolic byproduct of cannabis in participants’ urine samples, which gave more accurate measurements of cannabis exposure.
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.