A new study adds to a growing body of evidence that indicates prenatal marijuana use can impact children’s development and behavior.
Children of women who use marijuana during or soon after pregnancy are twice as likely as other kids to become anxious, aggressive or hyperactive, according to a new study. The findings add weight to a growing body of evidence linking cannabis use during pregnancy to psychiatric problems in children. The behavioral issues may be driven in part by changes in the activity of genes found in the placenta, the organ that provides nutrients and oxygen to the growing fetus.
For pregnant women, cannabis isn’t just a means of getting high. Some women use it to ease severe morning sickness and anxiety, and they may not be aware that it can pose risks.
Women “tend to think smoking and drinking during pregnancy need to be avoided at all costs, but not cannabis,” said Yoko Nomura, a behavioral neuroscientist at Queens College, City University of New York, and a co-author of the new study. “We have a long way to go to educate pregnant women, policymakers and even OB-GYN doctors on this issue.”
Research suggests that a growing number of women are using cannabis during pregnancy: One study found that in 2016, nearly twice as many women in California reported using cannabis while pregnant than in 2009.
Yet for more than 40 years, research has been raising concerns about the effects of marijuana use during pregnancy. A longitudinal study that began in 1978 linked maternal cannabis use with children’s behavioral problems as well as deficits in language comprehension, visual perception, attention and memory. More recent research has linked cannabis use in pregnancy to low birth weight, reduced IQ, autism, delusional thoughts and attention problems, although some other studies have not identified such associations.
Of concern, too, is that cannabis today is nothing like the cannabis of years past. Levels of THC, the compound responsible for most of marijuana’s psychological effects, have increased significantly in recent years. “One joint today is like 17 joints in the 1970s,” said Dr. Darine El-Chaâr, a maternal-fetal medicine physician at The Ottawa Hospital in Canada who studies the health risks of marijuana use during pregnancy and was not involved in the new study.
Studies have shown that THC can pass through the mother’s bloodstream to the placenta and then to the fetus. This is the case no matter how the cannabis is consumed, whether from smoking it, eating it or being exposed to it through vapors, oils or creams. If they contain THC, “they’re all going to pass through to the baby,” Dr. El-Chaâr said.
In the new study, Dr. Nomura and her colleagues at the Icahn School of Medicine at Mount Sinai in New York and the City University of New York asked 322 healthy mothers who had given birth between 2011 and 2015 at Mount Sinai Hospital or NewYork-Presbyterian, Queens, in New York City whether they had ever used cannabis during the pregnancy or in the 18 months after. Then they studied their children over time. The researchers gave the kids neurobiological tests each year to track their development, took and studied their hair samples, and gave the mothers tests to ascertain whether their children had behavioral issues.
The children of mothers who said they had used marijuana were more than twice as likely as children of abstinent mothers to be anxious, aggressive or hyperactive when they were between the ages of 3 and 6. Their hair also contained higher levels of the stress hormone cortisol, and they had heart rate variations that “have been linked with a number of anxiety-related disorders,” said a co-author of the study, Yasmin Hurd, the director of the Addiction Institute of Mount Sinai.
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.