The investigators conducted a study to evaluate the effect of nitrous oxide as an adjunctive to external cephalic version and we observed a slight decrease in patient reported pain (visual analogous scale), with no effect on the rate of success. Currently, in the protocol of Cruces University Hospital the administration of nitrous oxide during the manoeuvres of the external cephalic version is routine practice. The investigators consider that the analgesic remifentanil is a better option for the external cephalic version than nitrous oxide by the combination of rapid onset of effect, with analgesic potency similar to regional anaesthesia and rapid metabolism that prevents their accumulation in tissues.