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Emergency contraception (EC) with levonorgestrel (LNG) is hypothesized to work by preventing fertilization and, therefore, to be ineffective once an ovum is fertilized. Researchers assessed the likelihood of LNG EC failures in relation to ovulation status in women (age range, 18–38; menstrual cycle day 5–25) who received EC pills (1.5 mg LNG within 120 hours of unprotected sex) at a family planning clinic in Chile. Serum luteinizing hormone, progesterone, and estradiol were measured within 1 hour of taking EC; participants also underwent vaginal examinations to determine presence of spermatozoa, and transvaginal ultrasound was performed to evaluate stage of the ovulatory cycle. Women were contacted at the anticipated date of their next mens…