Drugs that relax the uterus (tocolytic agents) are sometimes used to help improve ECV success rates. Nitroglycerin is a tocolytic agent, but intravenous nitroglycerin has not been tested as an agent to help ECV. There is some suggestion that nitroglycerin may be more helpful in women who have not previously been pregnant (nulliparous women) than in women who have been pregnant more than once (multiparous women), and so we have planned two trials.