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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鈥攕uch as pregnant people鈥攖hat 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 麻豆学生精品版 have found that especially low birth weight, and that higher exposure is associated with higher risks.
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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 publishes online on December 12 in JAMA.
鈥淐annabis use is not safe,鈥 says Robert Silver, M.D., professor of obstetrics and gynecology at U of U Health and last author on the study. 鈥淚t increases the risk of pregnancy complications. If possible, you shouldn鈥檛 use cannabis during pregnancy.鈥
The researchers were driven to answer this question in part by the contradictory answers that many people encounter when trying to learn about the health impacts of cannabis use. 鈥淭here鈥檚 so much information out there鈥攄iscussion and social media channels and on the Internet鈥攁bout cannabis use and pregnancy,鈥 explains Torri Metz, M.D., vice chair of research of obstetrics and gynecology at U of U Health and lead author on the study. 鈥淚 think it鈥檚 hard for patients to understand what they should be worried about, if anything.鈥
Uncovering new risks
Indeed, some previous studies on the topic found no association between cannabis use and pregnancy complications. One hurdle facing such research, Metz says, is that there are 鈥渟o many differences between baseline characteristics of people who use and don鈥檛 use cannabis during pregnancy. There鈥檚 different rates of anxiety and depression.鈥 These differences can also impact pregnancy risks, which makes it challenging to figure out the consequences related specifically to cannabis use.
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鈥 urine samples, which gave more accurate measurements of cannabis exposure.
Open questions
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.
While this type of study can鈥檛 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. The correlation Metz and her colleagues observed suggests that cannabis may disrupt the human placenta in a similar way.
Silver adds that the greater risk seen at higher levels of exposure is especially concerning given the high amount of THC found in newer cannabis products鈥攑roducts 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 urge 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 have been proven to be safe. 鈥淭here are many, many reasons people use cannabis,鈥 Silver says. 鈥淏ut there may be alternative therapies that can help mitigate the symptoms.鈥
Silver emphasizes that continued research on the health impacts of cannabis is urgently needed so that patients can make informed decisions about their health. 鈥淎s long as humans are interested in using this product,鈥 he says, 鈥渨e ought to assess health effects both good and bad, as accurately as we can, and provide that information for folks.鈥
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The research published as and was carried out in collaboration with researchers from ARUP Laboratories, University of California, Irvine, The Ohio State University, Indiana University, Case Western Reserve University, University of Pennsylvania, Columbia University, Eastern Virginia Medical School, and University of Pittsburgh.
Support was provided by the National Institutes of Health and the Center for Clinical and Translational Sciences.