Association between Prenatal Marijuana and Tobacco Smoke Exposures and Small for Gestational Age at Birth

The aim of this study was to determine the association of prenatal marijuana exposure with and without tobacco smoke exposure and small for gestational age (SGA) at birth.

We conducted a secondary analysis of the prospective Lifestyle and Early Achievement in Families (LEAF) cohort enrolled from 2010 to 2015. We included singleton nonanomalous liveborn pregnancies. We assessed marijuana use inclusive of any pregnancy urine specimen with a Δ9-THC-COOH concentration of more than 15 ng/mL by mass spectrometry, self-report on questionnaire, and/or electronic health record; and self-reported maternal tobacco use. Because of the high co-frequency of marijuana with tobacco exposure in pregnancy and the known association between tobacco and fetal growth restriction, we modeled the exposure as: both marijuana and tobacco (hereafter "co-use"), only marijuana, only tobacco, and neither (reference). Incidence of SGA in each group was compared with the neither group. The primary outcome was SGA less than 10th percentile, and secondarily less than 5th percentile, using parity-specific definitions per 2017 US natality reference data.

Among 325 assessed mothers, 46% had neither exposure, 11% had only prenatal marijuana exposure, 20% only tobacco exposure, and 23% co-use exposure. A third (33%) of infants were SGA less than 10th percentile and 20% SGA less than 5th percentile. Marijuana exposure only was associated with an increased risk of SGA less than 10th percentile (43 vs. 26%; adjusted relative risk [aRR]: 1.66; 95% confidence interval [CI]: 1.02-2.69), and SGA less than5th percentile (30 vs. 13%; aRR: 2.26; 95% CI: 1.15-4.47). Tobacco was not associated with SGA less than 10th percentile, but was with SGA less than 5th percentile (26 vs. 13%; aRR: 2.01; 95% CI: 1.13, 3.56). Co-use was not associated with increased SGA risk in multivariate analysis, but was in sensitivity analysis when tobacco use was defined using a cotinine assay rather than self-report (SGA <10th percentile, aRR: 1.97; 95% CI: 1.24-3.15) and (SGA <5th percentile, aRR: 2.03; 95% CI: 1.09-3.78).

Prenatal marijuana exposure in addition to tobacco may increase the risk of SGA. Given the rising prevalence of marijuana use in pregnancy, further research is warranted to understand how in utero marijuana exposure may impact fetal growth and birth weight with and without tobacco exposure.

· Marijuana and tobacco are commonly used together in pregnancy.. · Prenatal marijuana and tobacco exposure may increase the risk of a small for gestational age infant.. · Further research is warranted to understand how in utero marijuana exposure impacts fetal growth..