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Published on Physicians Practice (http://www.physicianspractice.com)
Emergency Contraception Prevents Fertilization, not
Implantation, Studies Show
July 13, 2011
By OBGYN.net Staff [1]
Recent research by members of the Population Council's International Committee for Contraception
Research (ICCR) and other scientists shows that emergency contraceptive pills appear to work by
interfering with ovulation, thus preventing fertilization of the egg.
Reprinted with permission of Population Council
NEW YORK (2 May 2005)--Recent research by members of the Population Council's International
Committee for Contraception Research (ICCR) and other scientists shows that emergency
contraceptive pills appear to work by interfering with ovulation, thus preventing fertilization of the
egg. They do not appear to disrupt postfertilization events, such as the implantation of a fertilized
egg in the uterus.
Emergency contraception prevents pregnancy most effectively when taken within 72 hours of
unprotected intercourse. The researchers studied levonorgestrel, a progestin widely used for regular
hormonal contraception that is also used for emergency contraception. Emergency contraception
has been the subject of heated debate. At issue is the method's mechanism of action: does it
prevent the meeting of egg and sperm, or does it prevent a fertilized egg from implanting in the
uterus? A method that allows the fertilization of an egg but prevents the fertilized egg from
implanting in the uterus may be considered abortifacient by some.
Over the past few years, reproductive physiologist Horacio B. Croxatto of the Chilean Institute for
Reproductive Medicine in Santiago, Chile, and his colleagues have studied the effects of
levonorgestrel on the reproductive cycles of female rats, monkeys, and humans. Croxatto and one of
his study partners, biomedical researcher Vivian Brache of PROFAMILIA in Santo Domingo, Dominican
Republic, are members of the ICCR.
Croxatto and his colleagues exposed female rats to very high doses of levonorgestrel at various
stages of their reproductive cycles, either before or after ovulation or before or after mating. The
researchers found that levonorgestrel inhibited ovulation totally or partially, depending on the timing
of treatment and the dose administered. However, the drug had no effect on fertilization or
implantation. This research was published in the May 2003 issue of the journal Contraception.
Next, Croxatto and his colleagues studied the effects of levonorgestrel given to Cebus monkeys
either before ovulation or postcoitally. The reproductive cycle of each animal was monitored by
ultrasound examination of the ovaries, vaginal smears, and measurements of blood hormone levels,
in order to time the administration of levonorgestrel. The researchers found that, when given before
ovulation, levonorgestrel was able to inhibit or postpone ovulation. Alternatively, when it was given
after mating-at a time when fertilization was believed to have occurred (on the basis of previous
monitoring)-the pregnancy rates observed were identical in cycles treated with levonorgestrel or
with a placebo. This indicates that levonorgestrel did not interfere with any postfertilization process
required for embryo implantation. This research was published in the June 2004 issue of the journal
Human Reproduction.
may become pregnant when they have intercourse in the five days before ovulation. This is because
sperm can live in the female reproductive system for up to five days. An egg, however, is usually
viable for only six to 12 hours after it is released. Croxatto, Brache, and their colleagues studied the
effects of levonorgestrel administered during this fertile preovulatory period of women's menstrual
cycles. The researchers used Plan BR, a levonorgestrel-containing emergency contraceptive product
marketed in the United States and Canada.
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Emergency Contraception Prevents Fertilization, not Implantation, Studies Sho
Published on Physicians Practice (http://www.physicianspractice.com)
Twenty-nine women in Santiago and 29 women in Santo Domingo were enrolled in the study. All of
the women were protected from pregnancy by tubal ligation or a nonhormonal intrauterine device.
The study was randomized, double-blind, and placebo-controlled. Women were treated with either a
placebo, a full dose of Plan B emergency contraception, or a half dose of the drug. They were
followed over several menstrual cycles and, by the end of the study, each had received all three of
these treatments, separated by resting cycles. The women were randomly assigned to receive the
treatments at specific times during the fertile preovulatory period, according to the diameter of the
leading ovarian follicle, as determined by ultrasound. The leading ovarian follicle is the structure that
ruptures to release the egg. In 82 percent of Plan B-treated cycles, follicles failed to rupture within
the five-day period following treatment (the maximum time span sperm would survive in the female
reproductive tract), or there was some significant abnormality in ovulation. These conditions
occurred in only 41 percent of placebo cycles. The rate of failed or abnormal ovulation that was
observed with Plan B treatment is identical with the estimated efficacy rate of Plan B emergency
contraception. Blood tests on these women indicated that Plan B influences ovulation by suppressing
the surge of luteinizing hormone (LH), the hormone that triggers ovulation.
"There is no doubt that fertilization would not have taken place in those women should they have
had intercourse prior to treatment," says Croxatto. "We conclude that the effects exerted by Plan B,
when it is taken before the onset of the LH surge, may fully explain the pregnancies averted by
emergency contraception. Failure to affect the LH surge, because treatment was begun too late in
the fertile preovulatory period, explains the 20 percent failure rate of this method. Our data
presented in this paper suggest that emergency contraception using levonorgestrel works by
disrupting ovulation, not by interfering with implantation." This research was published in the
December 2004 issue of the journal Contraception.
The May 2005 Population Briefs is now available at
http://www.popcouncil.org/pdfs/popbriefs/pbmay05.pdf
Other articles in this issue are:
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Postabortion Complications Prevalent in Pakistan
Globalization Is Transforming Adolescence in the Developing World
Guide for Improving Adherence to Drug Therapies
Sperm with Bent Tails Point to Possible Male Contraceptive
Population Briefs highlights the Population Council's research in biomedicine, public health, and
social science as well as its international collaborations. The free newsletter is available in print and
electronically.
The Population Council (www.popcouncil.org) is an international, nonprofit, nongovernmental
organization that seeks to improve the well-being and reproductive health of current and future
generations around the world and to help achieve a humane, equitable, and sustainable balance
between people and resources. The Council conducts biomedical, social science, and public health
research and helps build research capacities in developing countries. Established in 1952, the
Council is governed by an international board of trustees. Its New York headquarters supports a
global network of regional and country offices.
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