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Population Council, May 2, 2005

Emergency Contraception Prevents Fertilization, not Implantation, Studies Show

NEW YORK --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.

Women 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 B®, a levonorgestrel-containing emergency
contraceptive product marketed in the United States and Canada.

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 woman 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 thatruptures 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.

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