Effectiveness of levonorgestrel emergency contraception given before or after ovulation — a pilot study

Novikova, Natalia; Weisberg, Edith; Stanczyk, Frank Z.; Croxatto, Horacio B.; Fraser, Ian S.

doi:10.1016/j.contraception.2006.08.015

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Volume 75, Issue 2 , Pages 112-118, February 2007

Effectiveness of levonorgestrel emergency contraception given before or after ovulation — a pilot study☆

Natalia Novikova
- Department of Obstetrics and Gynaecology, University of Sydney, NSW, 2006, Australia
Edith Weisberg
- Department of Obstetrics and Gynaecology, University of Sydney, NSW, 2006, Australia
- Sydney Centre for Reproductive Health Research, Division of Research, FPA Health, Sydney, NSE, 2006, Australia
Frank Z. Stanczyk
- Reproductive Endocrine Research Laboratory, Keck School of Medicine, University of Southern California, Los Angeles, CA 90033, USA
Horacio B. Croxatto
- Instituto Chileno de Medicina Reproductiva, Correo 22, Casilla 96, Santiago, Chile
Ian S. Fraser
- Department of Obstetrics and Gynaecology, University of Sydney, NSW, 2006, Australia
- Sydney Centre for Reproductive Health Research, Division of Research, FPA Health, Sydney, NSE, 2006, Australia
- Corresponding author. Tel.: +61 2 9351 2478; fax: +61 2 9351 4560.

Received 3 July 2006; received in revised form 28 August 2006; accepted 29 August 2006. published online 30 October 2006.

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: Emergency contraception, Pregnancies, Ovulation, Timing of intercourse, Timing of emergency contraception