The study does not, however, prove that prenatal cannabis use caused the children’s behavioral problems. Some of the mothers said they had used cannabis only after giving birth (though THC can pass through breast milk). And women who use cannabis may differ from abstinent women in other ways that put their children at risk for behavioral issues. They may have underlying risk factors, such as a family history of psychiatric problems, or they may have been exposed to other chemicals during pregnancy that increase the risk for behavioral issues in their kids, said Ryan Bogdan, a psychologist at Washington University in St. Louis who studies the underlying biology of psychiatric disorders. The authors of the study tried to control for such differences using statistical methods, but some could nevertheless have influenced the results.
Yet the researchers also identified possible biological explanations for their findings, which bolsters the possibility that marijuana is a driving factor. As part of the study, the researchers collected placentas from some of the mothers after they gave birth and analyzed the activity of their placental genes. They found that, compared with placentas from abstinent mothers, placentas from mothers who had used cannabis displayed dampened activity in genes that make key immune-related proteins. THC is known to suppress a marijuana user’s immune system, but it’s noteworthy to see these immune changes in the placenta, too, where they could directly affect the developing fetus.
Immune abnormalities and infections during pregnancy have long been linked with neuropsychiatric problems in kids, Dr. Hurd said. In women who catch the flu during their first trimester of pregnancy, for instance, some studies have found an increased risk of giving birth to a baby who later develops schizophrenia. Key immune chemicals made by the body to fight infections, known as cytokines, play important roles in brain signaling, so disturbances to these chemicals during pregnancy could affect brain development.
Marijuana could affect how the placenta feeds the growing fetus, too, Dr. Bogdan said. This could help explain why babies born to mothers who use cannabis during pregnancy are more likely to have low birth weight, and these nutrient restrictions could also affect the fetus’s brain development. Moreover, the placenta contains a protein that binds to THC and affects how neurons move and connect to one another, Dr. Bogdan added.
To tease out exactly what might be happening, we need more research, Dr. El-Chaâr said. Many questions don’t yet have clear answers, such as whether a certain amount of marijuana might be safe during pregnancy, or if any exposure could have potential effects on a growing baby. For now, especially in light of the new findings, she urges caution.
“What I would tell patients is that there’s no known safe amount,” she said.
Melinda Wenner Moyer is a contributing editor at Scientific American magazine and a regular contributor to The New York Times, The Washington Post and other publications. Her first book, “How to Raise Kids Who Aren’t Assholes,” was published in July.
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