Prenatal exposure to tobacco and marijuana can affect infant size and behavior, according to a study published in Child Development. Infants who were exposed to both substances, especially during the third trimester, were smaller in length, weight, and head size, in addition to a greater likelihood of being born early, compared to infants who were not exposed to the two substances.
The study included a sample of 247 mother-infant dyads. Of these, 173 had either been exposed to just tobacco or a combination of tobacco and marijuana. The mothers in the study ranged in age from 19 to 40 (M=25.39). Maternal race was 52% African American, 30% Caucasian, and 8% other or mixed race. Approximately 44% of the mothers were married or living with their partner, 35% were in a relationship but not cohabiting, and 21% were single. Approximately 26% of the mothers had less than a high school education, 60% completed high school, 10% completed some college courses, and 4% had a vocational degree or technical training degree.
Maternal smoking status was determined through a combination of self-report and maternal saliva samples collected once per trimester. Saliva was tested by the US Drug Testing Laboratory for ∆9-tetrahydrocannabinol (THC), the psychoactive component of marijuana. After the infant’s birth, meconium specimens were collected from soiled diapers twice daily until the appearance of milk stool, which was tested for THC and nicotine.
The researchers found that infants with positive meconium results for both tobacco and marijuana had smaller head circumference (mean difference [M]=32.79 cm, SD=2.06), shorter birth length (M=48.61 cm, SD=3.65), lower gestational age (M=38.36 weeks, SD =2.21) and birth weight (M=2886 g, SD=576.7) compared to infants who had negative meconium results for both substances ([head circumference, M=34.42 cm, SD =1.62], [birth length, M=50.35 cm, SD=1.96], [gestational age, M=39.2 weeks, SD=1.41], [birth weight, M=3340 g, SD=532.8]). Infants who tested positive for only tobacco had smaller head circumferences compared to infants who tested negative for both (M=33.63, SD=1.76).
In addition to testing for prenatal exposure to tobacco and marijuana, the researchers also analyzed maternal anger and prenatal/postnatal stress and infant reactivity and regulation. Symptoms of maternal anger were measured using the Buss-Perry Aggression Questionnaire, which was administered during the third trimester and again when infants reached 2 months. The participants self-reported and the researchers averaged the results from the four measured scales of physical aggression, verbal aggression, anger, and hostility. Higher scores indicated higher anger. Perceived stress was assessed using the Perceived Stress Scale (5-point scale), which was also administered once during the third trimester and once when infants reached 2 months. Again, the participants self-reported and the researchers extrapolated a mean value from the results. Infant reactivity and regulation (Infant RR) was measured at 9 months using the Infant Behavior Questionnaire-Revised as well as behavioral measures through an anger frustration paradigm.
Mothers who were positive for both tobacco and marijuana also reported high aggression in pregnancy and postpartum compared to those who were negative for both. The researchers used the mean scores from the self-reported questionnaires to determine mean values (M) and standard deviation (SD).
For tobacco- and marijuana-positive mothers, mean value recorded for anger during pregnancy was 3.01, SD=0.64 and for stress during pregnancy was 24.88, SD=7.25 versus 24.60, SD-8.15 for mothers not exposed to the two substances. After pregnancy, mothers who tested positive for both substances recorded postnatal mean scores for anger of 2.66, SD=0.74 and postnatal stress of 19.63, SD=7.17. Negative-testing mothers recorded mean values for postnatal anger of 2.31, SD=0.64 and for postnatal stress of 22.21, SD=8.46. While there was no direct significant association between co-exposure to tobacco and marijuana and infant RR, there was an indirect association between the two substances and lower birth weight and size (ß=.05, 95% CI [.01, .10]).
The researchers highlighted a few strengths and limitations of the study. They noted that while accurate assessment of substance use is difficult, they incorporated multiple indices of substance use into the study, including self-reporting, interviews, and maternal saliva and infant meconium samples. They also recognize that the study did not account for maternal psychological symptoms and personality disorders, which have been associated with cigarette smoking. Infant RR was also limited to a single brief task and thus, must be generalized. The researchers believe one of the study’s major strengths was controlling for alcohol exposure. Going forward, the authors said their findings illustrate the need to focus on helping pregnant smokers reduce stress and cope with negative emotions during cessation interventions.
Note from Dr. Lockwood
"We should always be mindful of the potential for reverse causality in such studies. For example, it is possible that anxious people are more likely to smoke cigarettes and marijuana."