Study links prenatal marijuana use to serious maternal complications including preeclampsia

A large study published this week in JAMA Internal Medicine has found that women who use marijuana before or during early pregnancy face substantially elevated risks of serious, sometimes life-threatening complications—a finding that arrives as cannabis use among pregnant women has more than doubled over the past two decades.

The research, led by Kelly Young-Wolff, a scientist at Kaiser Permanente Division of Research in California, examined health records from more than 250,000 pregnant people in Northern California. Participants were asked about marijuana use when they entered prenatal care around eight to ten weeks of pregnancy, and many also underwent urine testing to detect any cannabis use in the previous month. The study identified more than 20,000 cannabis users, including nearly 11,000 women who had not disclosed use but tested positive on urinalysis.

The numbers are striking. Pregnant people who used cannabis had a 17 percent greater risk of gestational hypertension—a dangerous elevation in blood pressure that typically develops after the first 20 weeks of pregnancy—and an 8 percent greater risk of preeclampsia, a more severe form of high blood pressure that can damage the liver, kidneys, and other organs. The study also found a 19 percent increased risk of placental abruption, a condition in which the placenta suddenly detaches from the uterine wall, depriving the fetus of oxygen and nutrients and endangering both mother and baby. Placental abruption ranks among the leading causes of death in both pregnant women and infants.

The risks intensified with frequency of use. Daily users faced the highest danger of gestational hypertension. Using marijuana at least once a month or more frequently was linked to greater risk of placental abruption. Young-Wolff noted that recent data shows an increasing proportion of pregnant cannabis users are now daily users—a concern amplified by the fact that modern marijuana is far more potent than it was decades ago. The study also found that pregnant marijuana users were 9 percent more likely to gain excessive weight during pregnancy and 5 percent more likely to gain too little.

Experts believe cannabinoids, particularly THC, bind to receptors in the placenta and disrupt estrogen signaling, affecting how the placenta develops and functions. Gestational hypertension can impede blood flow to the liver, kidneys, brain, uterus, and placenta. When it progresses to preeclampsia, protein appears in the urine, liver enzymes rise, blood platelet counts drop, and fluid can accumulate in the lungs. In severe cases, preeclampsia can trigger blood clots, seizures, stroke, kidney failure, and liver damage. Dr. Deborah Ansley, medical director of Kaiser Permanente's Early Start prenatal program, noted that preeclampsia can also force preterm delivery and neonatal intensive care admission, separating mother and baby at birth and disrupting the critical early bonding period.

Brianna Moore, an assistant professor at the Colorado School of Public Health who studies cannabis in pregnancy but was not involved in this research, called it the largest study to date examining cannabis use in pregnancy and adverse maternal health outcomes. She emphasized that it is never too late for pregnant women to reduce or stop cannabis use to minimize potential harm, and urged them to speak with their healthcare providers.

Young-Wolff pointed to a troubling gap between perception and reality. Pregnant women, she said, are receiving inaccurate messages from social media, cannabis retailers, and peers suggesting that marijuana use during pregnancy is safe. The study provides what she called timely evidence that cannabis use in pregnancy carries real danger. The researchers were careful to isolate their findings: they conducted a sensitivity analysis limited to pregnant patients who tested negative for other substances, confirming that the results were not attributable to use of other drugs.

The study does have limitations. Researchers could not determine whether marijuana use continued beyond the first trimester, could only test for THC rather than the more than 100 other cannabinoids present in cannabis, and could not distinguish between smoking and edible consumption. Because all participants were in Northern California—a state that legalized medical cannabis in 1996 and adult use in 2018—the findings may not apply to other regions. Participants were also older, had higher incomes, and nearly all had private insurance, which may limit how broadly the results apply to the general population.

Experts are urging pregnant women to discuss any cannabis use with their healthcare providers without fear of legal consequences. Young-Wolff recommended that those considering marijuana to manage pregnancy symptoms discuss those symptoms with their doctor and pursue clinically recommended treatments instead. For those unwilling to quit, reducing frequency of use remains an option worth pursuing.