Dr Brancazio’s tautologic observation is of course correct; in the absence of multiple prospective, randomized, double-blinded, placebo-controlled crossover trials, or the fulfillment of Koch’s postulates, it is difficult to categorically prove a cause-and-effect relationship in medicine. Unfortunately, this shortcoming applies to most treatments currently in use in clinical obstetrics. However, when one applies a technique proven to reduce pulmonary embolism by about 70% in most other areas of surgery to cesarean delivery, and finds about a 70% reduction in deaths from pulmonary embolism in the years immediately following this change with an N >1 million in study and control groups, it is reasonable to infer a causal connection, particularly when available diagnostic and therapeutic tools were the same in both time periods.