How do levonorgestrel‐only emergency contraceptive pills (LNG ECPs) prevent pregnancy?

Implantation: • One study has estimated effectiveness of LNG ECPs by confirming the cycle day by hormonal analysis (other studies used women's selfreported cycle date). In this study, pregnancies occurred only in women who took ECPs on or after the day of ovulation, while no pregnancies occurred in the women who took ECPs before ovulation, providing evidence that ECPs were unable to prevent implantation [8]. • A number of studies have evaluated whether ECPs produce changes in the histological and biochemical characteristics of the endometrium. Most studies show that LNG ECPs have no such effect on the endometrium, indicating that they have no mechanism to prevent implantation [1,2,9]. One study found a single altered endometrial parameter only when LNG was administered prior to the LH surge, at a time when ECPs inhibit ovulation [10]. • One study showed that levonorgestrel did not prevent the attachment of human embryos to a simulated (in vitro) endometrial environment [11]. • Animal studies demonstrated that LNG ECPs did not prevent implantation of the fertilized egg in the endometrium [12,13].