Use of serum evaluation of contraceptive and ovarian hormones to assess reduced risk of pregnancy among women presenting for emergency contraception in a multicenter clinical trial.

Many participants in a recent clinical trial of oral emergency contraception did not appear to be at risk for pregnancy or would not have benefited from intervention due to cycle timing. Investigators should consider the effects of these findings on expected pregnancy rates when determining sample size in future EC clinical trials, particularly when using noninferiority designs or historical controls.