In Colorado after legalization, 5.7% of women used cannabis during pregnancy and 5% of breastfeeding mothers used it postnatally, with prenatal use associated with a 50% increased likelihood of low birth weight.
Read this if you are pregnant or planning pregnancy and want to understand the evidence on cannabis and birth outcomes.
50% increased risk of low birth weight with prenatal cannabis use, independent of tobacco
What the researchers found
Researchers analyzed data from 3,207 Colorado women who completed the Pregnancy Risk Assessment Monitoring System survey with state-added cannabis questions. In a state where both medical and recreational marijuana were legal, 5.7% reported using cannabis at any point during pregnancy and 5% of breastfeeding women used it in the early postnatal period.
Prenatal cannabis use was associated with a 50% increased likelihood of delivering a low birth weight baby, even after controlling for maternal age, race/ethnicity, education level, and tobacco use during pregnancy. This independent association was statistically significant.
However, small for gestational age, preterm birth, and neonatal intensive care unit admission were not significantly associated with prenatal cannabis use after controlling for tobacco use. This suggests that while cannabis may affect fetal growth, it may not independently increase the risk of these other adverse outcomes.
Why it matters
Colorado was one of the first states to legalize recreational marijuana, making it a natural laboratory for studying the effects of legalization on perinatal cannabis use. The finding of a significant association with low birth weight in a population-based sample provides important evidence for prenatal counseling and screening.
The numbers in context
3,207 Colorado women surveyed. Prenatal cannabis use: 5.7%. Postnatal use among breastfeeding mothers: 5.0%. Low birth weight: 50% increased likelihood with prenatal cannabis (OR 1.5, 95% CI 1.1-2.1, p = 0.02). No significant association with preterm birth, SGA, or NICU admission after controlling for tobacco.
How the study worked
Cross-sectional analysis of 3,207 respondents from the 2014-2015 Colorado PRAMS survey with state-developed cannabis questions. Multiple logistic regression evaluated relationships between prenatal cannabis use and neonatal outcomes (low birth weight, small for gestational age, preterm birth, NICU admission), controlling for maternal demographics and tobacco use.
What this study cannot tell us
Self-reported cannabis use likely underestimates true prevalence. The cross-sectional design limits causal conclusions. Cannabis use was measured dichotomously (any use during pregnancy) without capturing dose, frequency, or timing. The survey could not distinguish between different cannabis products or potencies. Residual confounding from unmeasured factors is possible.
How to read the evidence
This is a population-based survey with appropriate statistical controls, providing moderate evidence from one of the first legalized states.
When this study was published
Published in 2018 with 2014-2015 data, early in Colorado's legalization experience.
The bigger picture
As cannabis legalization expands, prenatal exposure is likely increasing. These population-level data from one of the first legalized states provide a baseline for understanding the reproductive health implications of legal cannabis access. The association with low birth weight, independent of tobacco, strengthens the case for universal prenatal cannabis screening.
Questions still open
- Does the amount and timing of prenatal cannabis use affect the severity of birth weight effects? Is the low birth weight association mediated by placental effects, nutritional changes, or direct fetal effects? Should universal cannabis screening be implemented in prenatal care?
Common questions
Is it safe to use cannabis during pregnancy?
How common is cannabis use during pregnancy?
Read the original research
Cannabis Use During the Perinatal Period in a State With Legalized Recreational and Medical Marijuana: The Association Between Maternal Characteristics, Breastfeeding Patterns, and Neonatal Outcomes.
The Journal of pediatrics, 197, 90-96
Citation
Crume, Tessa L; Juhl, Ashley L; Brooks-Russell, Ashley; Hall, Katelyn E; Wymore, Erica; Borgelt, Laura M. (2018). Cannabis Use During the Perinatal Period in a State With Legalized Recreational and Medical Marijuana: The Association Between Maternal Characteristics, Breastfeeding Patterns, and Neonatal Outcomes.. The Journal of pediatrics, 197, 90-96. https://doi.org/10.1016/j.jpeds.2018.02.005
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