“The current study combined with the existing literature about the long-term consequences of prenatal cannabis and tobacco exposure support the importance of preventing and reducing smoking cannabis and cigarettes during pregnancy,” the authors, led by Hanan El Marroun, Department of Child and Adolescent Psychiatry, Erasmus University, Rotterdam, the Netherlands, write.
The study was published online June 15 in Biological Psychiatry.
Tobacco vs Cannabis Use
Researchers recruited study participants from the Generation R Study, an ongoing population-based prospective cohort study. Children aged 6 to 8 years were invited to participate in an MRI component.
Prenatal cannabis exposure was measured with maternal self-report as well as through urinalysis. Maternal tobacco use was prospectively assessed using postal questionnaires sent during each trimester.
The study included three groups: 113 nonexposed children; 96 children whose mothers smoked only tobacco during pregnancy; and 54 children whose mothers used cannabis during the pregnancy.
The authors note that prenatal cannabis use commonly co-occurs with smoking during pregnancy. In this study, only 14.8% of cannabis users did not smoke tobacco during pregnancy; 74.1% continued smoking during pregnancy.
Results showed that compared with nonexposed control persons, tobacco-exposed children, but not cannabis-exposed children, had smaller global brain volume.
Compared with nonexposed children, those exposed to cannabis had thicker frontal cortices, specifically, a thicker superior frontal area of the left hemisphere (clusterwise P< .001) and a thicker frontal pole of the right hemisphere (clusterwise P = .003).
One possible explanation for this increase in thickness of the prefrontal cortex is an alteration in neurodevelopmental maturation, the authors note.
“The prefrontal cortex is the most rostral portion of the neocortex and is involved in cognitive functions,” they write. “It is one of the higher-order cortical regions to undergo later maturation as compared with regions that are associated with lower-order cortices such as the somatosensory and the visual cortices.”
The prefrontal cortex supports functions such as the ability to suppress responses and thoughts, attention, higher-order motor control, and working memory.
The association between prenatal cannabis exposure and thicker cortices remained significant after correcting for covariates that included maternal education, household income, marital status, ethnicity, alcohol use, maternal psychopathology, and the child’s IQ and birth weight.
The authors had previously shown that prenatal cannabis exposure was associated with increased aggression and problems with attention in offspring, particularly in girls. As the children become older, this association may become more evident, they note.
Other studies have also linked prenatal cannabis exposure to lower scores on assessments of language, memory, and abstract/visual reasoning.
Findings from these studies “have led to the hypothesis that prenatal cannabis use may have selective deleterious consequences on developing executive functions,” the authors write.
Prenatal exposure to tobacco exposure was associated with thinner cortices in the left and right hemisphere, namely, the superior parietal (clusterwise P < .001) and superior frontal regions (P < .001). The association remained statistically significant after accounting for covariates.
A limitation of the study was that brain morphology was assessed at only one time point, making it impossible to infer conclusions about the trajectory of neurodevelopment in study participants. Also, cannabis use was assessed only once during pregnancy, and self-reported information about cannabis use in the second and third trimesters was not available.
The high degree of cannabis and tobacco use together makes it very difficult to conclude that the observed effects in the cannabis-exposed children were driven by cannabis exposure only, the authors write.
More research is needed to explore the causal nature of the association between prenatal cannabis exposure and brain morphology, they conclude.
The study was supported by the Sophia Children’s Hospital Fund, the Erasmus Medical Centre, and the Netherlands Organization for Health Research and Development. The authors have disclosed no relevant financial relationships.
Biol Psychiatry. Published online June 16, 2016