Cannabis Use in Pregnancy Linked to Small Rise in Psychotic-Like Behaviours in Children, Study Finds

The study at the center of this story looked at 11,489 children, and 655 of them had been exposed to THC before birth. Published in JAMA Psychiatry, the cross-sectional analysis found that those children showed a small elevation in psychotic-like behaviours. The behaviours included aggression toward others, along with attention and social problems.

The obvious objection is that women who already have certain behavioural traits might be more likely to use cannabis while pregnant. The researchers considered that, along with other confounding factors such as genetic predispositions, and the relationship held. Whether it is causal is a separate question. The design cannot answer it, and there may be factors the researchers did not account for. Still, set against other research, the link is worth pursuing.

The study's design rested on timing. Animal models suggest THC acts through endocannabinoid receptors in the developing brain, and these do not appear in the human fetus until five or six weeks into gestation. That is roughly when many women learn they are pregnant. So the authors focused on people who kept using cannabis after they knew. Children exposed only before their mothers knew of the pregnancy showed no differences from children never exposed. That pattern supports the idea that those receptors play a role in development. Other lines of evidence have also tied prenatal cannabis exposure to shorter attention span and some behavioural problems.

The backdrop is rising use. A 2019 study of more than 450,000 pregnant women found cannabis use more than doubled between 2002 and 2017, reaching 7 percent, making it the most popular illicit drug among pregnant women. Cannabis is reportedly used against nausea and vomiting in early pregnancy, but there is little evidence on whether it works or is safe.

Much remains unstudied. Evidence suggests fetal THC concentrations can differ depending on whether the mother uses monthly or daily, but dosage and timing during pregnancy have barely been examined. Researchers have also not properly compared edibles with burning flower. There is currently no known safe level of cannabis use during pregnancy or lactation. The American College of Obstetricians and Gynaecologists and the American Academy of Pediatrics both advise against use in early pregnancy or while breastfeeding, and so does the US Surgeon General. His announcement notes that THC has been found in breast milk for up to six days after last use, and that it may affect a newborn's brain development and result in hyperactivity and poor cognitive function.

That message appears not to be landing. Surveys show pregnant patients do not feel their doctors give them useful information about cannabis, and some have turned to the internet. One of the most widely shared sources is a small 1990s study of pregnant women in Jamaica, whose positive results showed no clear cause and effect. It was based on cannabis far weaker in THC than many plants today.

Larger studies since have found more worrying results. A Canadian study of more than 660,000 pregnancies tied cannabis use to greater risk of preterm birth, placental abruption, intensive care admission, and lower birth weight and Apgar scores. Some effects looked dose dependent: women using at least weekly had roughly a 5 percent greater chance of preterm delivery than those using less often. A US study linked prenatal use to a 50 percent increased likelihood of low birth weight.

Alcohol and tobacco carry well-documented pregnancy risks, but many women do not know weed may carry risks too. ACOG's analysis says that, given concerns about impaired neurodevelopment and exposure to the effects of smoking, women who are pregnant or contemplating pregnancy should be encouraged to stop using marijuana. Until dose, timing and method are studied, that remains the standing advice.