Emergency contraception (EC) does not always work. Clinicians should be aware of potential risk factors for EC failure. Data from a meta-analysis of two randomized controlled trials comparing the efficacy of ulipristal acetate (UPA) with levonorgestrel were analyzed to identify factors associated with EC failure. The risk of pregnancy was more than threefold greater for obese women compared with women with normal body mass index (odds ratio (OR), 3.60; 95% confidence interval (CI), 1.96-6.53; p<.0001), whichever EC was taken. However, for obese women, the risk was greater for those taking levonorgestrel (OR, 4.41; 95% CI, 2.05-9.44, p=.0002) than for UPA users (OR, 2.62; 95% CI, 0.89-7.00; ns). For both ECs, pregnancy risk was related to the cycle day of intercourse. Women who had intercourse the day before estimated day of ovulation had a fourfold increased risk of pregnancy (OR, 4.42; 95% CI, 2.33-8.20; p<.0001) compared with women having sex outside the fertile window. For both methods, women who had unprotected intercourse after using EC were more likely to get pregnant than those who did not (OR, 4.64; 95% CI, 2.22-8.96; p=.0002). Women who have intercourse around ovulation should ideally be offered a copper intrauterine device. Women with body mass index >25 kg/m(2) should be offered an intrauterine device or UPA. All women should be advised to start effective contraception immediately after EC. Copyright © 2011 Elsevier Inc. All rights reserved.