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

Area of Science:

• Reproductive Endocrinology
• Contraception Research
• Hormonal Contraceptives

Background:

• Subdermal implants containing levonorgestrel (NORPLANT) are a long-acting reversible contraceptive method.
• Understanding the mechanisms of action, including effects on ovulation and luteal function, is crucial for assessing contraceptive efficacy.
• Previous studies have suggested hormonal contraceptives can alter ovulatory patterns.

Purpose of the Study:

• To investigate the roles of anovulation and inadequate luteal function in the contraceptive action of NORPLANT.
• To assess the impact of NORPLANT on ovulation and corpus luteum function over a 7-year period.
• To correlate levonorgestrel levels with observed effects on reproductive cycles.

Main Methods:

• Plasma progesterone levels were measured in 113 women using NORPLANT implants and 22 women using Copper T 200 IUDs.
• Blood samples were collected multiple times over 7 years, with progesterone measured via radioimmunoassay (RIA).
• Samples were categorized as ovulatory, uncertain, or anovulatory based on established laboratory criteria.

Main Results:

• Anovulation rates in NORPLANT users ranged from 25% to 80% over 7 years, peaking in the first year.
• Anovulation rates were significantly correlated with plasma levonorgestrel levels.
• Inadequate luteal function was detected in varying degrees among NORPLANT users, while the control group maintained normal ovulatory cycles.

Conclusions:

• Continuous levonorgestrel administration via NORPLANT significantly interferes with ovulation and/or luteal function in a substantial proportion of cycles.
• These disruptions in ovulatory and luteal function are key components of NORPLANT's contraceptive mechanism.
• The findings highlight the dynamic impact of hormonal implants on female reproductive physiology throughout their duration of use.