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Is Smoking Marijuana During Pregnancy Harmful?

Stay away from marijuana during pregnancy.

As the recreational use of marijuana becomes increasingly accepted (and legal), it is important to be aware of various risks associated with this drug. For instance, does marijuana use during pregnancy influence the developing child?

Healthcare providers should inform women of childbearing age about the risks versus benefits of their medications should they become pregnant. Some medications may pose risks to the fetus; however, there may be risks to both mother and fetus if a medication is discontinued. For instance, different medications used to treat bipolar disorder can have varied risks to the developing fetus. These risks need to be considered in the context of the consequences to the fetus and the pregnant woman should relapse of the bipolar disorder occur. Such decisions are best made following discussion with a physician knowledgeable about the specific medications and the specific course of the illness in the individual.

How does one evaluate the risks versus benefits of recreational drugs? First of all, what are the benefits? Many find marijuana use enjoyable, but otherwise the benefits are debatable. But are there risks to the fetus? Before addressing this question, it is important to review how marijuana influences the human brain.

Marijuana stimulates specific cannabinoid receptors that are plentiful in the brain. These receptors normally interact with two cannabinoid neurotransmitters called anandamide and 2-arachidonyl glycerol (2AG). In adults, this endogenous cannabinoid system is involved in several critically important bodily functions including energy regulation, i.e., the intake of calories. It turns out that the cannabinoid system in the brain is also very active during fetal development.

Although studying the effects of marijuana during human pregnancy is difficult, several studies in animals have shown that prenatal and perinatal exposure to cannabis results in various deficits in offspring. A paper by G.A. Vargish and colleagues in the journal Molecular Psychiatry examines the influence of marijuana on in utero development of mice. The endogenous cannabinoid system in mouse pups is similar to that in human fetuses.

These investigators studied a part of the brain involved in learning and memory known as the hippocampus. This region has a very well defined structure that lends itself to detailed anatomical and electrophysiological study.

Vargish and colleagues found that injecting pregnant mice with THC, the active ingredient in marijuana, led to substantial decreases in a specific cell type known as interneurons. The loss of these cells was associated with marked changes in the electrical functioning of the neural circuits in the hippocampus. The structural and functional changes were correlated with diminished social functioning when the mouse pups were older.

This paper, together with earlier research, suggests that marijuana use during pregnancy is likely to have deleterious consequences to the developing fetus. As this issue becomes increasingly studied, more specific understanding of the potential harm to developing children will be clarified.

Bottom line: If marijuana is being used for recreational purposes, it is best to stop using it during pregnancy. It is also important to emphasize that marijuana is not unique in this regard. Exposure to numerous other drugs including alcohol, nicotine, and certain therapeutic agents during pregnancy is associated with problems in neurodevelopment that have well-documented, long-lasting adverse behavioral and cognitive consequences.

This column was written by Eugene Rubin MD, PhD and Charles Zorumski MD.


Vargish, G.A., Pelkey, K.A., Yuan, X., Chittajallu, R., Collins, D., Fang, C., & McBain, C.J. (2017). Persistent inhibitory circuit defects and disrupted social behaviour following in utero exogenous cannabinoid exposure. Molecular Psychiatry. 22: 56-67.