A Prospective Study of Preconception and Early Pregnancy Cannabis Use and Birth Outcomes

BACKGROUND: Cannabis use among reproductive-aged individuals is common, yet there are gaps in our understanding of the effect of cannabis use on adverse birth outcomes. OBJECTIVE: To determine the extent to which female and male preconception cannabis use and female early pregnancy cannabis use are associated with the incidence of preterm birth (PTB), small-for-gestational-age birth (SGA), and large-for-gestational-age (LGA) birth. METHODS: We analysed data from Pregnancy Study Online, an internet-based preconception cohort study of couples trying to conceive. Eligible participants were aged 21-45 years (female) and ≥ 21 years (male) and residents of the United States or Canada. We followed 6838 couples who conceived between June 2013 and September 2024. Both partners completed preconception baseline questionnaires; female participants completed preconception follow-up questionnaires every 8 weeks, at early (< 12 weeks) and late pregnancy (~32 weeks), and at 6 months postpartum. Participants self-reported cannabis use during preconception (female and male) and early pregnancy (female only). We ascertained birthweight and gestational age from birth certificates and questionnaires and defined PTB (delivery < 37 gestational weeks) and SGA and LGA (< 10th and > 90th percentiles of birthweight, respectively, for gestational age and sex). We fit Cox regression models to estimate hazard ratios (HR) and 95% confidence intervals (CI), adjusting for potential confounders. RESULTS: Preconception cannabis use was not appreciably associated with PTB (≥ 1 per week vs. non-use: HR 1.06, 95% CI 0.71, 1.57 for females and HR 1.41, 95% CI 0.73, 2.75 for males). Higher frequency of female cannabis use in early pregnancy was associated with an increased incidence of PTB, although results were imprecise (≥ 1 per week vs. non-use: HR 1.87, 95% CI 1.06, 3.29; < 1/week vs. non-use: HR 0.85, 95% CI 0.36, 2.02). Cannabis use in either partner was not associated with the incidence of SGA or LGA. CONCLUSIONS: Our results support the existing evidence that cannabis use during early pregnancy is a risk factor for PTB.