Abstract
Objective 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.
Study Design 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.
Results 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).
Conclusion 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.
Key Points
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Marijuana and tobacco are commonly used together in pregnancy.
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Prenatal marijuana and tobacco exposure may increase the risk of a small for gestational
age infant.
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Further research is warranted to understand how in utero marijuana exposure impacts
fetal growth.
Keywords
marijuana - tobacco - smoke - small for gestational age at birth - pregnancy
