Levonorgestrel EC has no effect after ovulation.
Emergency contraceptive pills containing levonorgestrel are not effective at preventing pregnancy if taken after ovulation, according to a study of 388 women aged 18-38 who requested emergency contraception at a Chilean Institute of Reproductive Medicine clinic in Santiago within 120 hours of unprotected sex. (1) Researchers determined by blood hormone levels and transvaginal ultrasound that 122 of the women had had unprotected intercourse during one of the six fertile days of their menstrual cycle (the five days before ovulation and the day of ovulation); 87 received emergency contraception before ovulation and 35 after. Among women who received emergency contraception prior to ovulation, the researchers were able to verify ovulation for 71%. Emergency contraception prevented pregnancy among women who received it before ovulation: Of the 13 pregnancies that would have been expected among women in this group in the absence of emergency contraception, none actually occurred. However, among women who received emergency contraception after ovulation, the expected and observed numbers of pregnancies were similar (seven and six, respectively). The authors conclude that levonorgestrel "is very effective in preventing pregnancy when it is administered before ovulation, but it is ineffective in preventing pregnancy once fertilization has occurred."
(1.) Noe G et al., Contraceptive efficacy of emergency contraception with levonorgestrel given before or after ovulation. Contraception, 2010, 81(5):414420.
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|Title Annotation:||UPDATE; emergency contraceptives|
|Publication:||International Perspectives on Sexual and Reproductive Health|
|Article Type:||Brief article|
|Date:||Dec 1, 2010|
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