Another issue with Dreher’s study, and any study of its age, is that cannabis potency has increased dramatically over time. “I would throw those studies out completely. I don’t think you could go back to those early studies and say they have many implications for what is going on today,” says Barry Lester, a professor of psychiatry and pediatrics and the director of the Center for the Study of Children at Risk at Brown University. In the U.S., the THC content of illicit cannabis samples increased from 4 percent in 1995 to 12 percent in 2014, and legal cannabis obtained in 2017 in Washington State was about 20 percent THC, with some products potentially reaching 30 percent or higher. There are little data on how this might affect fetal exposure, but a 2015 abstract described an increase in the concentration of THCA (a THC metabolite) measured in the first bowel movements of Colorado newborns from 213 nanograms per gram in 2012 to 361 in 2014.
The cannabis used by the women in Dreher’s study wasn’t high in THC, she says. “It was not anything like the cannabis that we have now.”
Lester also collaborated with Dreher in Jamaica in the 1980s, studying the same cohort of babies from her more well-known paper. His study, published with Dreher as a co-author in Child Development in 1989, found that at four and five days of age, the cannabis-exposed newborns had shorter, higher-pitched cries, with more dysphonation and variability. These were the same babies with no difference in behavior at three days old, but Lester says that didn’t surprise him, because the behavioral scales used aren’t typically designed to be as sensitive. “Cry acoustics has a long-standing track record of differentiating at-risk kids,” he says. “If you pick a dependent measure that you know has that kind of sensitivity, you’re obviously more likely to find effects.”
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Yet for all their criticism of Dreher’s study, these researchers also wanted to talk about the limitations of the current science and the need for nuanced interpretation.
Because it would be unethical to randomly assign women to use cannabis while pregnant, the evidence we have on this question is observational and rife with confounding factors. In North America, those who use cannabis are also more likely to use tobacco, alcohol, and other drugs, and to face the challenges that come with poverty, mental illness, and the effects of racism. Researchers try to statistically account for these differences (indeed, each of these factors was identified and accounted for in the Ontario JAMA study), but quantifying all the inequities affecting mothers and their children is a tall order, and a real limitation for the existing evidence.
That adds uncertainty to researchers’ understanding of the health risks of cannabis, says Jarlenski, the University of Pittsburgh researcher. Still, she rattled off a host of other reasons not to use cannabis while pregnant, including unstandardized potency, the lack of evidence of benefits, the risk of dependency, and the unknowns of how it might interact with postpartum depression. There’s also a risk of being reported to child-welfare services for using a substance that’s still considered a Schedule I drug at the federal level, and poor women of color are likely at greatest risk of this consequence.