Fifteen women with regular menstrual periods and seven amenorrheic women who had been using a levonorgestrel-releasing (LNg) IUD for more than seven years were studied. For controls, eight women using TCu380Ag IUDs for more than seven years were studied during two complete menstrual cycles. Ovarian function was assessed with hormonal determination and ultrasound examinations. The regularly menstruating women were studied for two complete menstrual cycles and the amenorrheic women for eight weeks. In the regularly menstruating LNg-IUD users, according to progesterone levels, 93% of the cycles were ovulatory but just 58% of these 'ovulatory' cycles showed normal follicular growth and rupture. Follicular cysts and luteinization of regressing follicles were observed in 42% of the 26 'ovulatory' cycles studied. SHBG capacity was decreased in the LNg-IUD users compared with the TCu380Ag users. Progesterone levels were lower in the LNg-IUD users compared with the TCu380Ag users but this difference was not statistically significant. Preovulatory estradiol and LH levels were lower in the LNg-IUD users than in the TCu380Ag users. These differences were not statistically significant. For the amenorrheic women, five had follicular cysts that disappeared spontaneously within 45 days. Two women showed follicular development and rupture. The presence of good cervical mucus was observed in 69% of the ovulatory cycles studied in the LNg-IUD users. This indicates that effects on cervical mucus cannot be the main mechanism of action of the LNg-IUDs. It is concluded that LNg-IUDs may exert a contraceptive effect in many different ways, such as inhibition of ovulation, endometrial changes preventing implantation, alteration of physical and chemical properties of cervical mucus affecting sperm transport and subtle disturbances in hypothalamic pituitary ovarian function, resulting in alterations of follicular development and rupture.

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