To what extent is research on cannabis in pregnancy complicated by tobacco? That is the question doctors at the Washington University in St. Louis sought to answer with a study published in 2016. Since women who smoke marijuana are more likely to smoke tobacco, a known teratogen (fetal toxin), research on cannabis in pregnancy might yield false positive results. To answer this question the doctors reanalyzed data from 31 different studies – every publication on cannabis and pregnancy that met a few basic criteria, like measuring low birth weight and preterm birth, as well as distinguishing polydrug-using mothers from cannabis-only mothers. Their conclusion was strong: “the association between maternal marijuana use and adverse pregnancy outcomes may be attributable to concomitant tobacco use and other confounding factors and not marijuana alone.” This may be surprising to those who have seen many studies suggesting cannabis reduces birth weight. But nearly every such study admits that they couldn’t exclude the effects of tobacco, alcohol, and other factors. So study after study ends up biased by the same problem. When these reports are aggregated in a meta-analysis, however, there is enough data to disentangle the effects of tobacco and cannabis. This unintuitive situation is called Simpson’s paradox: many small experiments show one result (i.e. cannabis is associated with low birth weight), but pooling data exposes underlying factors (i.e. tobacco use). Once these extra factors are controlled, the association between cannabis use and low birth weight vanishes. These results, of course, don’t prove that cannabis absolutely can’t be harmful in pregnancy. But they indicate that, on it’s own, cannabis use has at most a very small effect, and that public health messages should be tailored towards women using tobacco and alcohol. Potential negative effects on long-term development couldn’t be assayed and should be studied in the future. But a lack of certainty is not an excuse to dredge up a nightmarish vision of children irreparably harmed from cannabis.