Prevalence of cannabis use among pregnant women is on the rise with the spread of legalization. The most frequently reported reasons for use are to relieve stress or anxiety, nausea or vomiting, pain, and for recreation.
Given that the primary psychoactive ingredient of cannabis, ∆9-tetrahydrocannabinol (THC) is a small fat-soluble molecule that can easily cross the placenta, an Israeli-U.S. research team conducted a systematic search of the peer-reviewed medical literature for studies exploring possible neuropsychiatric effects on offspring.
They included not only studies evaluating likelihood of ADHD, but also autism spectrum disorder, anxiety, depression, and psychotic symptoms. For each of these, adjustment was made for known confounding variables.
With that adjustment, meta-analysis of six studies with a total of over half a million (503,661) participants reported a 13% increase in the odds of ADHD in offspring of mothers using cannabis during pregnancy compared with offspring of mothers not using cannabis while pregnant.
That is generally considered a small effect size increase in risk. But there are multiple reasons to question even this minimal finding:
- It barely achieved statistical significance.
- A few studies used more reliable clinical diagnoses, while most just used ADHD symptom rating scales.
- It is virtually impossible to eliminate all confounding variables. Twin studies come closest to fully accounting for otherwise unmeasured environmental and genetic confounders, but no such studies were included.
- The team made no effort to evaluate publication bias.
- Almost all the participants (497,821) were in a single study, and that study – which relied on clinical diagnoses – did not find a significant association.
Meta-analysis of two studies with a total of 741 individuals reported a 20% increase in offspring use of cannabis among mothers who used cannabis during pregnancy, but once again this was subject to methodological shortcomings:
- Two studies do not make for a robust meta-analysis, even more so with only 741 participants.
- The result barely achieved statistical significance.
- Publication bias was unaddressed.
- Small effect sizes are questionable due to the virtual impossibility of eliminating all confounding variables, especially without twin studies.
Some studies have suggested a link between cannabis and psychotic symptoms. But meta-analysis of four studies combining over nineteen thousand persons found no significant association between maternal cannabis use during pregnancy and offspring psychotic symptoms.
Many studies have pointed to commonalities in the etiology of ADHD and autism spectrum disorder (ASD). Yet meta-analysis of five studies encompassing over half a million participants found absolutely no association between maternal prenatal cannabis use and ASD.
The remaining meta-analyses also reported no association with depression or anxiety.
With the caution that absence of evidence is not the same as evidence of absence, it is by no means clear from what is presently known that prenatal cannabis exposure has any significant neuropsychiatric effects on offspring. And if further research finds any effects, they are likely to be minor.