On the mechanisms of action of short-term levonorgestrel administration in emergency contraception.

The effects of short-term administration of levonorgestrel (LNG) at different stages of the ovarian cycle on the pituitary-ovarian axis, corpus luteum function, and endometrium were investigated. Forty-five surgically sterilized women were studied during two menstrual cycles. In the second cycle, each women received two doses of 0.75 mg LNG taken 12 h apart on day 10 of the cycle (Group A), at the time of serum luteinizing hormone (LH) surge (Group B), 48 h after positive detection of urinary LH (Group C), or late follicular phase (Group D). In both cycles, transvaginal ultrasound and serum LH were performed from the detection of urinary LH until ovulation. Serum estradiol (E2) and progesterone (P(4)) were measured during the complete luteal phase. In addition, an endometrial biopsy was taken at day LH + 9. Eighty percent of participants in Group A were anovulatory, the remaining (three participants) presented significant shortness of the luteal phase with notably lower luteal P4 serum concentrations. In Groups B and C, no significant differences on either cycle length or luteal P4 and E2 serum concentrations were observed between the untreated and treated cycles. Participants in Group D had normal cycle length but significantly lower luteal P4 serum concentrations. Endometrial histology was normal in all ovulatory-treated cycles. It is suggested that interference of LNG with the mechanisms initiating the LH preovulatory surge depends on the stage of follicle development. Thus, anovulation results from disrupting the normal development and/or the hormonal activity of the growing follicle only when LNG is given preovulatory. In addition, peri- and post-ovulatory administration of LNG did not impair corpus luteum function or endometrial morphology.

Agentes antiimplantaçäo / Anti-implantation agents

Anti-implantation agents ara characterized as drugs or methods that have their contraceptive effects between fertilization and the end of the impllantation period (days 13-14 post-fertilization). Such agents con be used im post-coital emergency, for repeated post-ovulatory use and/or for menstrual regulation process. Menstrual regulation can be achived by antiorigestins that block progesterone receptors and interfere with the preparation of the endometrium for implantation, by luteolysis causing decreased progesterone levels and interruption of the implantation process, by the uterotonic effects of prostaglandinas. RU 486 administered during yhe luteal phase induces vaginal bleeding ind both women and non-human primates and prevention of prgenancy might be possible by their use as a once-a-month menstrual regulator. As yet, no post-coital agent has been satisfactorily identified and other Anti-implantation agents are being considered