Skip to main content

Verified by Psychology Today


Pregnancy and Cannabis Are a Bad Recipe

Indications of adverse consequences of prenatal cannabis exposure are mounting.

Key points

  • Cannabis use is common during pregnancy, especially in the first trimester.
  • THC concentrations in a fetus are one-third of the concentration found in the mother's blood.
  • THC interferes with a fetus's natural cannabinoid chemistry guiding axon growth necessary for proper brain development.
  • Adverse consequences for cognition and attention throughout childhood and adolescence stem from prenatal cannabis exposure.

Cannabis use during pregnancy is common, especially among women accustomed to using cannabis prior to becoming pregnant. In 2012, average cannabis use during the three trimesters of pregnancy was reported to be 9 percent, 5 percent, and 2.4 percent, respectively.

Unfortunately, the first trimester is a critical period for laying down the basic foundation of our nervous system. Half of the women using cannabis during pregnancy do so to relieve nausea and vomiting, which is worst during the first trimester. Women who believe cannabis is a safe treatment for morning sickness likely don’t know that THC concentrations in the fetus are approximately one-third of what is measured in the mother’s blood. As many as 28 percent of young, urban, disadvantaged women use cannabis during pregnancy. Prenatal exposure to cannabis is widespread, and fortunately, no major birth defects are known to result from prenatal exposure to cannabis.

Maternal cannabis use is important because fetal nerve cell migration, axonal wiring, and the development of synapses are all guided by our natural cannabinoid chemistry. Anandamide and 2-AG levels are five times higher in fetal brains than in those of adults, and they activate cannabinoid receptors (CB1) at the leading edge of axonal growth to build internal microtubular scaffolding that directs neurons to their proper destination. Introducing THC at this critical period of fetal brain development causes branching in this scaffolding, decreasing axonal integrity. A protein critical to microtubular growth (SCG10) has been shown to be reduced by 20 percent in human fetuses electively aborted during the second trimester and known to have been exposed to cannabis. Without this protein, axons are not properly guided in their growth.

Only a few scientists have investigated the impact of cannabis exposure on fetal brain development. Long-term studies, beginning with an assessment of cannabis use during pregnancy and continuing for the first two decades of a child’s life, are difficult, expensive, and require persistent dedication throughout a researcher’s career. Such studies are therefore rare. Only three have been conducted for long enough to provide data, but they differ so greatly in their methodology and populations studied that none of their results have been independently validated. One in Ottawa studied middle-class women; one in Pittsburgh studied disadvantaged women; and the third was in Rotterdam, Netherlands. Nevertheless, these studies clearly show that there are a variety of measurable cognitive deficits throughout childhood, adolescence, and early adulthood in individuals exposed to second-hand cannabis while still in their mother’s womb.

Deficits include impaired mental development at 9 months, impaired abstract reasoning and short-term memory at 3 years old, impaired memory and verbal development at 4 years old and impaired attention and impulse control with hyperactivity at 6 years old. When higher cognitive abilities begin to mature in the child and can be more reliably tested, documented adverse consequences of prenatal cannabis exposure include increased hyperactivity, impulsivity, and inattention at 10 years old, and impaired executive function and problem-solving at 9-12 years old.

Adverse consequences in adolescence include impaired problem-solving and analytic skills that require sustained attention, observed in 13- to 16-year-olds. From ages 18 to 22, fMRIs performed during working memory and executive function tasks reveal significantly less activity in the frontal lobes and compensatory increased activity in multiple other areas of the cortex.

These results clearly reveal a pattern of difficulties with attention and executive functions throughout childhood and adolescence. Only future research will establish the full extent of the problems caused by prenatal cannabis exposure. Recently released data from an ongoing landmark study of nearly 12,000 children across the USA (the Adolescent Brain Cognition and Development (ABCD) Study) confirm the findings of the three previous long-term studies of the negative impact of cannabis on the fetal brain. Despite the accumulating evidence, fetuses are still being exposed to even higher levels of THC today than were available when previous studies began.

Will the subtle findings reported by early studies become more obvious over the next two decades in newborns exposed to today’s higher levels of THC? Are we in only the early stages of recognizing an expanding problem, one that will look obvious in the future? This prospect led the American College of Obstetrics and Gynecology to recommend that the only prudent course of action is to avoid cannabis use during pregnancy. And the American Academy of Pediatrics warns, “Women who are considering becoming pregnant or who are of reproductive age … particularly adolescents and young adult women … need to be informed about … potential adverse effects of THC use on the woman and on fetal, infant, and child development;] .… marijuana should not be used during pregnancy.”

The preceding post is abstracted from my book, Marijuana on My Mind: The Science and Mystique of Cannabis.

More from Timmen L. Cermak MD
More from Psychology Today
More from Timmen L. Cermak MD
More from Psychology Today