Evidence review raises concern about cannabis use in pregnancy

An updated systematic review finds that consuming cannabis while pregnant appears to increase the odds of preterm birth, low birth weight and infant death.

The study by researchers at Oregon Health & Science University published today in the journal JAMA Pediatrics.

The lead author is a physician-scientist who provides prenatal care for high-risk pregnancies at OHSU.

“Patients are coming to me in their prenatal visits saying, ‘I quit smoking and drinking, but is it safe to still use cannabis?’” said lead author Jamie Lo, M.D., M.C.R., associate professor of obstetrics and gynecology (maternal-fetal medicine) in the OHSU School of Medicine. “Until direct harms have been proven, they perceive it to be safe to use.”

In fact, cannabis remains one of the most common substances used in pregnancy that’s still illegal under federal law, and, unlike declines in prenatal use of alcohol or nicotine, cannabis use is continuing to increase. Lo said many of her patients are reluctant to give up cannabis during pregnancy because it helps to reduce common prenatal symptoms such as nausea, insomnia and pain.

Researchers updated the systematic review and meta-analysis, drawing on a total of 51 observational studies involving 21.1 million people to examine the potential adverse effects of cannabis use in pregnancy. The researchers found eight new studies since their previous update, raising the certainty of evidence from “very-low-to-low” to “moderate” for increased odds of low birth weight, preterm birth and babies being small for their gestational age.

The updated review also indicated increased odds of newborn mortality, though still with low certainty.

Researchers noted that the new systematic review includes a larger proportion of human observational studies examining people who only use cannabis, but don’t also use nicotine. And even though the evidence is low to moderate for adverse outcomes, Lo noted that the findings are consistent with definitive evidence in nonhuman primate models exposed to THC, the main psychoactive compound in cannabis.

The related research in animal models included standard prenatal ultrasound and MRI imaging that revealed a detrimental effect on the placenta, in terms of blood flow and availability of oxygen in addition to decreased volume of amniotic fluid.

“These findings tell me as an obstetrician that the placenta is not functioning as it normally would in pregnancy,” Lo said. “When the placenta isn’t functioning well, it can affect the baby’s development and growth.”

Even though cannabis remains a Schedule 1 substance under the federal Controlled Substances Act, Oregon is one of several states that have legalized it under state law for medicinal and recreational use. Lo said she recommends a harm-reduction approach to patients. For those who cannot abstain, she advises them to reduce the amount and frequency of use to help reduce the risk of prenatal and infant complications.

“Even using less can mitigate the risk,” she said. “Abstinence is ideal, but it’s not realistic for many patients.”

In addition to Lo, co-authors include Snehapriya Yeddala, PharmD., Beth Shaw, M.S., Shannon Robalino, M.S., of OHSU; Chelsea Ayers, M.P.H., Rachel Ward, B.A., of the Veterans Affairs Portland Health Care System; and Devan Kansagara, M.D., of OHSU and the Portland VA.

The research is part of the Systematically Testing the Evidence on Marijuana (STEM) project, funded by the Office of Rural Health in the Veterans Health Administration of the U.S. Department of Veterans Affairs. Research is also supported by the National Institute on Drug Abuse of the National Institutes of Health, award DP1 DA056493. The content is solely the responsibility of the authors and does not necessarily represent the official views of the VA or the NIH.