The progestative effect of the levonorgestrel-releasing intrauterine system--when does it manifest?

The objective of the study was to evaluate, clinically and sonographically, the time required for the progestative effect of the levonorgestrel-releasing intrauterine system (IUS, Mirena) to be manifested. Doppler flow of the cervical branch and spiral artery of the uterine artery, as well as the endometrial width (up to day 10 of the cycle), were evaluated in 36 women carrying levonorgestrel-releasing IUS 1-2 months after insertion of the device compared to 4-6 months after insertion. The rate of intermenstrual bleeding was reduced from 44% during the first 2 months, to only 8% of women after 4-6 months of use. Complete cessation of menstrual bleeding occurred in 5% after 2 months and in 66% after 4-6 months following insertion. While there was no change in the Doppler flow in the cervical branch of the uterine artery between both groups, there was a significant reduction in the subendometrial flow in the spiral artery. This observation was reinforced by the demonstration of significant reduction in endometrial thickness in the following 4 months of use. The present study has demonstrated that the local progestative effect of the levonorgestrel-releasing IUS on the endometrium is already manifested within a period of 3 months and over after insertion.