Effectiveness of levonorgestrel emergency contraception given before or after ovulation--a pilot study.
BACKGROUND
Although widely used, the mechanisms of action of the levonorgestrel emergency contraceptive pill (LNG ECP) are still unclear. There are increasing data to indicate that LNG is particularly effective as an ECP by interrupting follicular development and ovulation. An important outstanding question is whether it has any effect on fertilization or implantation.


METHODS
Ninety-nine women participated; they were recruited at the time they presented with a request for emergency contraception. All women took LNG 1.5 mg in a single dose during the clinic consultation. A blood sample was taken immediately prior to ingestion of the ECP for estimation of serum LH, estradiol and progesterone levels to calculate the day of ovulation. The specimens were analyzed in a single batch. Based on these endocrine data, we estimated the timing of ovulation to be within a +/-24-h period with an accuracy of around 80%. Women were followed up 4-6 weeks later to ascertain pregnancy status. The effectiveness of ECP when taken before and after ovulation was determined.


RESULTS
Three women became pregnant despite taking the ECP (pregnancy rate, 3.0%). All three women who became pregnant had unprotected intercourse between Days -1 and 0 and took the ECP on Day +2, based on endocrine data. Day 0 was taken as ovulation day. Among 17 women who had intercourse in the fertile period of the cycle and took the ECP after ovulation occurred (on Days +1 to +2), we could have expected three or four pregnancies; three were observed. Among 34 women who had intercourse on Days -5 to -2 of the fertile period and took ECP before or on the day of ovulation, four pregnancies could have been expected, but none were observed. We found major discrepancies between women's self-report of stage of the cycle and the dating calculation based on endocrine data.


CONCLUSIONS
These data are supportive of the concept that the LNG ECP has little or no effect on postovulation events but is highly effective when taken before ovulation.
Novikova
Natalia
N
Department of Obstetrics and Gynaecology, University of Sydney, NSW, 2006, Australia.
Weisberg
Edith
E
Stanczyk
Frank Z
FZ
Croxatto
Horacio B
HB
Fraser
Ian S
IS
D000284:Administration, Oral; D000293:Adolescent; D000328:Adult; D001315:Australia; D003271:Contraceptive Agents, Female / Q000008:administration & dosage*; D003280:Contraceptives, Oral, Synthetic / Q000008:administration & dosage*; D003281:Contraceptives, Postcoital / Q000008:administration & dosage*; D004958:Estradiol / Q000097:blood; D005260:Female; D006801:Humans; D016912:Levonorgestrel / Q000008:administration & dosage*; D008597:Menstrual Cycle / Q000097:blood / Q000187:drug effects; D010060:Ovulation / Q000097:blood / Q000187:drug effects*; D010865:Pilot Projects; D011247:Pregnancy; D018873:Pregnancy Rate; D011374:Progesterone / Q000097:blood
D003271:Contraceptive Agents, Female; D003280:Contraceptives, Oral, Synthetic; D003281:Contraceptives, Postcoital; D011374:Progesterone; D004958:Estradiol; D016912:Levonorgestrel