On causal inferences from retrospective and observational studies and their implications for neuraxial labor analgesia: the CHRISTMAS* study.

Background: Observational studies should not be used to infer causation as they are prone to confounding factors, selection bias, and reverse causality. Many observational studies of labor analgesia treated epidurals as an independent exposure and concluded that "epidurals" cause dystocia, despite multiple randomized controlled trials showing no effect. We highlight this problem using reductio ad absurdum.