New Study: Smoking Marijuana During Pregnancy May Double Your Baby’s Risk of Autism

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The thinking may go something like this: Marijuana is legal. It must be safe. Maybe it’s a good way to manage stress, pain, or morning sickness during pregnancy. That thinking could not be farther from the truth. Marijuana smoking during pregnancy has already been linked to stillbirth, low birthweight, and a  higher risk of hyperactivity disorder in children. Unfortunately, many pregnant women are not getting the message. In fact, marijuana use during pregnancy has doubled since 2002.

Now, the largest study ever done to look at brain development in children of mothers who smoke marijuana during pregnancy has discovered another reason not to use marijuana, autism. A study from Canadian researchers finds that women who use marijuana during pregnancy may increase their child’s risk for autism by 51 percent!

The Study

The study is published in the August 2020 issue of the journal Nature Medicine. The researchers reviewed the birth registries of women in Ontario, Canada, from 2007 to 2012. Information on marijuana use during pregnancy was available for over 170,000 mothers. Health records of the children were also reviewed.

Of these women, over 2,000 women who reported marijuana use were compared to women of similar age, education, and socioeconomic status, who did not use marijuana. Women who smoked cigarettes, drank alcohol, or used opioids along with marijuana during pregnancy were excluded.

This study suggests that smoking marijuana during pregnancy decreases a child’s intellectual ability and increases the risk for a learning disorder by about 25 percent. However, the biggest risk was for autism. Autism can be diagnosed by age 2, and is usually diagnosed by age 4. During the time of the study, marijuana use causes a considerable increase in Autism diagnosis.

The results are not shocking to autism researchers, who have known from animal studies that marijuana disrupts brain development. Many women who use marijuana use it in the early part of pregnancy, thinking it may be safer. In fact, that is the worst time to subject a developing brain to marijuana. Animal studies show that brain receptors for marijuana are present by 5 to 6 weeks of development.


Some of other the risks for autism are known, but the link to marijuana is new and significant. Known risk factors include being a boy, having a family history of autism, older age of parents, and preterm birth. Vaccinations are not a risk factor or cause of autism. According to the CDC, autism has been increasing and may affect as many as one in 68 children. This study suggests that the increased number of women using marijuana during pregnancy could be contributing to the increased number of children being diagnosed. Adding autism to the other risks of marijuana during pregnancy makes the decision easy. Don’t use marijuana while pregnant. Full stop.

Christopher Iliades
Dr. Chris Iliades is a medical doctor with 20 years of experience in clinical medicine and clinical research. Chris has been a full time medical writer and journalist since 2004. His byline appears in over 1,000 articles online including EverydayHealth, The Clinical Advisor, and Healthgrades. He has also written for print media including Cruising World Magazine, MD News, and The Johns Hopkins Children's Center Magazine. Chris lives with his wife and close to his three children and four grandchildren in the Boston area.

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