To examine the association of prenatal cannabis use and adverse infant outcomes in a nationally representative cohort and consider the impact of concurrent cigarette exposure.

We conducted a retrospective cohort study on 32 583 new mothers from the 2017-2019 Pregnancy Risk Assessment Monitoring System. Cannabis use was evaluated as a binary variable (use or no use) as well as ordinal categories (no, light, moderate, heavy use). We used multivariable logistic regression to examine the relationship between prenatal cannabis exposure and low birthweight (LBW), preterm birth, and small for gestational age.

Prenatal cannabis use was associated with significantly greater odds of LBW (aOR, 1.27; 95% CI, 1.05-1.54) and small for gestational age (aOR, 1.35; 95% CI, 1.09-1.68) but not preterm birth. Compared with nonusers, heavy users (weekly or more) were twice as likely to deliver a LBW infant (aOR, 2.07; 95% CI, 1.46-2.94) or small for gestational age infant (aOR, 2.14; 95% CI, 1.38-3.30). When examining combined cannabis and cigarette use, prenatal exposure to both substances increased the likelihood of LBW (aOR, 2.27; 95% CI, 1.71-3.01), preterm birth (aOR, 1.61; 95% CI, 1.12-2.31), and small for gestational age (aOR, 3.29; 95% CI, 2.39-4.55) compared with no use, and the increased odds were greater than for either substance alone.

Our results suggest that cannabis use during pregnancy may harm fetal development, and recommendations to improve birth outcomes should address co-use of cannabis and tobacco.