The levonorgestrel intrauterine system: a clinical perspective from the UK.

The main contraceptive effects of this valuable product [the levonorgestrel intrauterine system (LNG-IUS)] are by endometrial suppression and changes to the cervical mucus and uterotubal fluid that impair sperm migration. The blood levels of LNG are very low, so progestogenic symptoms are uncommon. Most women still ovulate and in the remainder sufficient estrogen for health is produced from the ovary, even if they become amenorrhoeic, as many do; this is primarily a local end-organ effect and should be seen as a benefit.