Which statement about marijuana use and preterm birth is supported by evidence?

• A. Marijuana use has not been independently related to preterm birth
• B. Marijuana use independently increases preterm birth risk
• C. Marijuana use reduces preterm birth risk
• D. Marijuana use is the primary predictor of preterm birth

Answer: (A)

The important idea is disentangling a true, independent effect from associations that are confounded by other risk factors. In studies of marijuana use and preterm birth, much of what looks like a risk in unadjusted analyses disappears once researchers account for confounders such as tobacco use, other substance use, socioeconomic factors, and maternal health conditions. When these factors are controlled for, marijuana exposure often does not show an independent link to preterm birth. This is why the best-supported statement is that marijuana use has not been independently related to preterm birth.

Some studies do note associations in crude analyses, and a few show small effects in certain populations, but those findings are not consistently observed after rigorous adjustment. Conversely, asserting a clear increase in risk, a protective effect, or that marijuana is the primary predictor would require a consistent, independent association across high-quality studies, which the current evidence does not robustly support.