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

Wesselink A.; Geller R.; Harlow A.; Ncube C.; Kuan K.; Savitz D.; Rothman K.; Eisenberg M.; Wise L. · Paediatric and perinatal epidemiology · 2026

**Background & Methods** This prospective cohort study analyzed data from 6,838 couples enrolled in the Pregnancy Study Online between June 2013 and September 2024 to examine associations between preconception and early pregnancy cannabis use and adverse birth outcomes. Female and male participants (aged ≥21 years) completed baseline questionnaires, with female participants completing additional follow-up assessments every 8 weeks throughout pregnancy and at 6 months postpartum. Birth outcomes (preterm birth, small-for-gestational-age, and large-for-gestational-age) were ascertained from birth certificates and maternal questionnaires, with Cox regression models used to estimate hazard ratios adjusted for potential confounders. **Key Findings** • Preconception cannabis use in either partner was not significantly associated with preterm birth risk (female ≥1/week: HR 1.06, 95% CI 0.71-1.57; male ≥1/week: HR 1.41, 95% CI 0.73-2.75). • Higher frequency of female cannabis use during early pregnancy (≥1 per week) was associated with increased preterm birth incidence (HR 1.87, 95% CI 1.06-3.29), though less frequent use showed no elevated risk (HR 0.85, 95% CI 0.36-2.02). • Cannabis use in either partner during preconception or early pregnancy was not associated with small-for-gestational-age or large-for-gestational-age births. **Dosage & Administration** Cannabis use frequency was self-reported and categorized as ≥1 per week versus non-use and <1 per week versus non-use. Specific routes of administration, cannabinoid concentrations, and THC/CBD ratios were not reported. **Safety & Adverse Effects** Direct adverse effects of cannabis use were not reported. The primary safety concern identified was the association between frequent early pregnancy cannabis use and preterm birth, suggesting potential fetal/neonatal complications rather than direct maternal side effects. **Evidence Quality** This study represents moderate-quality evidence with several limitations: self-reported cannabis use introduces potential recall and social desirability bias; imprecise confidence intervals around the early pregnancy preterm birth association limit conclusions; lack of detailed cannabinoid composition data restricts mechanistic understanding; and attrition bias may have occurred given the internet-based cohort design. The prospective design and detailed longitudinal follow-up strengthen causal inference compared to retrospective studies. Results are consistent with existing evidence suggesting early pregnancy cannabis use increases preterm birth risk, but larger studies with objective biomarkers are needed to confirm causality.

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