Plasma progesterone levels during long-term treatment with levonorgestrel silastic implants

Plasma progesterone levels were measured in 113 women using a subdermal implant containing 200 mg levonorgestrel (NORPLANT(TM)) in an attempt to assess the roles of anovulation and inadequate luteal function in the contraceptive action of this method. Twenty-two women using an IUD (Copper T 200) served as a control group. Blood samples were taken twice a week for 6 consecutive weeks. This was done one to four times in each subject during the first 7 years after insertion of NORPLANT. Plasma progesterone was measured by a specific RIA and criteria based upon normal values in this laboratory were used to define each set of samples as ovulatory, uncertain or anovulatory. The rate of anovulation varied between 25 and 80% throughout the 7 years. The highest rate was observed during the first year of treatment. The rate of anovulation was significantly correlated with the plasma levels of levonorgestrel. Varying degrees of inadequate corpus luteum function were detected among NORPLANT users. All sets of samples in the control group were ovulatory and compatible with normal luteal function. These results clearly show that continuous administration of levonorgestrel by means of NORPLANT interferes with ovulation and/or luteal function in a significant proportion of cycles throughout the first 7 years of use.