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

Abstract

Plasma progesterone levels were measured in 113 women using a subdermal implant containing 200 mg levo-norgestrel (NORPLANT) in an attempt to assess the roles of anovulation and inadequate luteal function in the contraceptive action of this method. Women (22) using an IUD [intrauterine device] (Cu T 200) served as a control group. Blood samples were taken twice a wk for 6 consecutive wk. This was done 1-4 times in each subject during the first 7 yr after insertion of NORPLANT. Plasma progesterone was measured by a specific RIA [radioimmunoassay] 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 yr. The highest rate was observed during the 1st yr of treatment. The rate of anovulation was significantly correlated with the plasma levels of levo-norgestrel. 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. Continuous administration of levo-norgestrel by means of NORPLANT interferes with ovulation and/or luteal function in a significant proportion of cycles throughout the first 7 yr of use.