The mechanism by which Norplant prevents conception is only partially explainer There are three probable modes of action, which are similar to those attributed to to contraceptive effect of the progestin-only minipills:
1. The levonorgestrel suppresses, at both the hypothalamus and the pituitary, the luteinizing hormone (LH) surge necessary for ovulation. As determined by progesterone levels in many users over several years, about one-third of all cycles are ovulatory.'5
2. The levonorgestrel has a marked effect on the cervical mucus. The mucus thickens and decreases in amount, forming a barrier to sperm penetration.
3. The constant level of levonorgestrel suppresses the estradiol-induced cyclic maturation of the endometrium and eventually causes atrophy. These changes could prevent implantation should fertilization occur; however, no evidence of fertilization can be detected in Norplant users.