Totality of the Evidence Suggests Prenatal Cannabis Exposure Does Not Lead to Cognitive Impairments : A Systematic and Critical Review
Doi : 10.3389/fpsyg.2020.00816
Background: Despite limited data demonstrating pronounced negative effects of prenatal cannabis exposure, popular opinion and public policies still reflect the belief that cannabis is fetotoxic.
Methods: This article provides a critical review of results from longitudinal studies examining the impact of prenatal cannabis exposure on multiple domains of cognitive functioning in individuals aged 0 to 22 years. A literature search was conducted through PsycINFO, PubMed, and Google Scholar. Articles were included if they examined the cognitive performance of offspring exposed to cannabis in utero.
Results: An examination of the total number of statistical comparisons (n = 1,001) between groups of participants that were exposed to cannabis prenatally and non-exposed controls revealed that those exposed performed differently on a minority of cognitive outcomes (worse on <3.5 percent and better in <1 percent). The clinical significance of these findings appears to be limited because cognitive performance scores of cannabis-exposed groups overwhelmingly fell within the normal range when compared against normative data adjusted for age and education.
Conclusions: The current evidence does not suggest that prenatal cannabis exposure alone is associated with clinically significant cognitive functioning impairments.
In the United States (U.S.), and in most countries around the world, cannabis is illegal. Still, according to recent data from the U.S., more than 25 million people reported past month cannabis use, easily out-pacing the number of current cocaine (2.2 million) and heroin users (494,000) (NSDUH Detailed Tables, 2017). In addition, 63.8% of respondents to a global drug survey endorsed using cannabis at least once, a rate higher than any other illicit drug (GDS, 2019). Taken together, these findings demonstrate that cannabis use persists in the U.S. as well as around the globe despite legal restrictions.
Recently, countries such as Uruguay and Canada have legalized cannabis for recreational purposes. In the U.S., 11 states have legalized adult cannabis use, while 33 states now allow medical use of the drug. As a result of these recent developments, increased concerns have been raised about cannabis use by pregnant individuals and the impact it may have on the developing fetus. Indeed, cannabis is the most frequently used illicit substance by reproductive aged women in the U.S. (van Gelder et al., 2010; NSDUH Detailed Tables, 2017; National Pregnancy Health Survey, 2019). However, reported use during pregnancy is uncommon (Ko et al., 2015; NSDUH Detailed Tables, 2017; National Pregnancy Health Survey, 2019). Even when cannabis is used by pregnant individuals, use of the drug substantially decreases as pregnancy progresses (Ko et al., 2015). Nonetheless, there remains a minority of women who consume cannabis throughout pregnancy (Ko et al., 2015).
There is a growing scientific database assessing the effects of prenatal cannabis exposure on a myriad of measures, including early physical growth. In general, when proper controls are included, no relationship between prenatal cannabis exposure and adverse physical neonatal outcomes such as birth weight and head circumference has been found (Conner et al., 2016; Grant et al., 2018). Still, a concern expressed in the scientific literature is that although cannabis may not lead to severe physical abnormalities in infants, it might cause subtle changes in the brain that later manifest as deficits in cognitive functioning.
A burgeoning number of reviews have assessed the impact of prenatal cannabis exposure on cognitive functioning (Karila et al., 2006; Wu et al., 2011; Calvigioni et al., 2014; Higuera-Matas et al., 2015). The studies reviewed show that subtle differences in the cognitive performance between children who had been exposed to the drug prenatally and controls do exist, but the conclusions drawn sometimes extend too far beyond the actual data. For example, based on these subtle differences, some researchers have suggested that children prenatally exposed to cannabis exhibit cognitive deficits and/or behavioral abnormalities (Karila et al., 2006; Wu et al., 2011; Calvigioni et al., 2014; Higuera-Matas et al., 2015). The clinical implications of these subtle differences, however, are nearly impossible to determine without knowledge of the expected range of performance for a particular group. Through the use of normative data, whereby individual or mean group scores are compared against a normative database that accounts for age, and educational level, the clinical significance of the differences can be determined. This is a core assessment principle in clinical neuropsychology but appears to be largely ignored in the literature on prenatal cannabis exposure (Harvey, 2012).
In light of the important caveat highlighted above, we felt a critical review of the empirical literature on the cognitive outcomes of children prenatally exposed to cannabis was warranted. In order to assess the clinical significance of findings from the studies reviewed, we determined whether data for cannabis-exposed groups fell outside the average range of functioning when compared against a normative database. If, study investigators did not compare their data with normative scores—this was the case for several studies—we made such comparison ourselves whenever possible. Thus, this article addresses an important gap in our scientific knowledge in that findings should shed light on the extent to which prenatal cannabis exposure produces clinical consequences on offspring. This, of course, could have important public health and policy implications.