levonorgestrel (LNG) MOA is 1. inhibition of ovulation T 2. prevent follicular rupture. T 3.cause luteal dysfunction T 4.LNG taken after luteinising hormone surge has been shown to result in ovulatory dysfunction. F Levonorgestrel (LNG) The precise mode of action of levonorgestrel (LNG) is incompletely understood but it is thought to work primarily by inhibition of ovulation.16,17 Administration of LNG appears to prevent follicular rupture or cause luteal dysfunction.16–21 LNG taken prior to the luteinising hormone surge has been shown to result in ovulatory dysfunction in the subsequent 5 days.17 LNG can thus inhibit ovulation for 5–7 days, by which time any sperm in the reproductive tract will have become non-viable. The closer to ovulation treatment is given, the less likely the probability of interfering with this process. Indeed LNG has been shown to be no better than placebo at suppressing ovulation when given immediately prior to ovulation17 and is not thought to be ffective once the process of fertilisation has occurred.22–25 Studies looking at the effect of LNG on endometrial markers of receptivity have found little to no effect using different modes of administration.16,26,27 Evidence from an in vitro study indicates that LNG does not affect embryo-endometrial attachment.28 The available evidence suggests that pregnancies occurring after LNG failure are not associated with any major congenital malformations, pregnancy complications or other adverse pregnancy outcomes.