Abstract
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.
Method
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.
Conclusion
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.
Keywords
To read this article in full you will need to make a payment
Purchase one-time access:
Academic & Personal: 24 hour online accessCorporate R&D Professionals: 24 hour online accessOne-time access price info
- For academic or personal research use, select 'Academic and Personal'
- For corporate R&D use, select 'Corporate R&D Professionals'
Subscribe:
Subscribe to ContraceptionAlready a print subscriber? Claim online access
Already an online subscriber? Sign in
Register: Create an account
Institutional Access: Sign in to ScienceDirect
References
- Postcoital contraception with 0.75 mg d-norgestrel (Postinor).Ther Hung. 1981; 29: 31-34
- Low dose mifepristone and two regimens of levonorgestrel for emergency contraception: a WHO multicentre randomised trial.Lancet. 2002; 360: 1803-1810
- Mechanism of action of emergency contraceptive pills.Contraception. 2006; 74: 87-89
- Mechanism of action of hormonal preparations used for emergency contraception: a review of the literature.Contraception. 2001; 63: 111-121
- Mechanisms of action of emergency contraception.Steroids. 2003; 68: 1095-1098
- On the mechanisms of action of short-term levonorgestrel administration in emergency contraception.Contraception. 2001; 64: 227-234
- Emergency contraception with mifepristone and levonorgestrel: mechanism of action.Obstet Gynecol. 2002; 100: 65-71
- Postcoital treatment with levonorgestrel does not disrupt postfertilization events in the rat.Contraception. 2003; 67: 415-419
- Post-coital administration of levonorgestrel does not interfere with post-fertilization events in the New-World monkey Cebus apella.Hum Reprod. 2004; 19: 1352-1356
- Ethinyl estradiol and conjugated estrogens as postcoital contraceptives.JAMA. 1980; 244: 1336-1339
- Normal variation in the length of the luteal phase of the menstrual cycle: identification of the short luteal phase.Br J Obstet Gynaecol. 1984; 91: 685-689
- Mifepristone (RU 486) compared with high-dose estrogen and progestogen for emergency postcoital contraception.N Engl J Med. 1992; 327: 1041-1044
- Safety and effectiveness of hormonal postcoital contraception: a prospective study.Eur J Contracept Reprod Health Care. 1999; 4: 27-33
- Estimating the efficacy of emergency contraception ��� how reliable are the data?.Contraception. 2002; 66: 19-22
- Randomised controlled trial of levonorgestrel versus the Yuzpe regimen of combined oral contraceptives for emergency contraception.Lancet. 1998; 352: 428-433
- Immulite reproductive hormone assays: multicenter reference range data.Diagnostic Products Corp, Los Angeles2000 (Document; No. ZB157-D)
- Timing of sexual intercourse in relation to ovulation. Effects on the probability of conception, survival of the pregnancy, and sex of the baby.N Engl J Med. 1995; 333: 1517-1521
- Postovulatory ageing of the human oocyte and embryo failure.Hum Reprod. 1998; 13: 394-397
- New estimates of the effectiveness of the Yuzpe regimen of emergency contraception.Contraception. 1998; 57: 363-369
- The risk of conception on different days of the menstrual cycle.Popul Stud. 1969; 23: 455-461
- Fecundability, coital frequency and the viability of ova.Popul Stud. 1980; 34: 397-400
- Using the ratio of urinary oestrogen and progesterone metabolites to estimate day of ovulation.Stat Med. 1991; 10: 255-266
- Likelihood of conception with a single act of intercourse: providing benchmark rates for assessment of post-coital contraceptives.Contraception. 2001; 63: 211-215
- Plasma steroid and gonadotrophin profiles in ovulatory but infertile women.Clin Endocrinol. 1978; 8: 241-255
- Hormonal profile of the cycle in 68 normally menstruating women.Acta Endocrinol. 1980; 94: 89-98
- Effectiveness of the Yuzpe regimen of emergency contraception by cycle day of intercourse: implications for mechanism of action.Contraception. 2003; 67: 167-171
- A prospective randomized comparison of levonorgestrel with the Yuzpe regimen in post-coital contraception.Hum Reprod. 1993; 8: 389-392
- A randomized trial of mifepristone (10 mg) and levonorgestrel for emergency contraception.Obstet Gynecol. 2004; 104: 1307-1313
Article info
Publication history
Published online: October 30, 2006
Accepted:
August 29,
2006
Received in revised form:
August 28,
2006
Received:
July 3,
2006
Footnotes
���Funding for this study came from The Family Planning Foundation, FPA Health, Sydney.
Identification
Copyright
© 2007 Elsevier Inc. Published by Elsevier Inc. All rights reserved.
