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Studies change views on how morning-after pill works

Scientists and political activists have long assumed that the emergency contraceptive Plan B interferes with the implantation of a newly fertilized embryo. As a result, many pro-life pharmacists are refusing to fill the prescriptions, leaving distraught customers at risk of pregnancy.

While supporters of ready access to Plan B point out that it does not harm an implanted embryo and, thus, does not terminate a pregnancy, they have not challenged the underlying assumption that Plan B causes changes to the uterus that can prevent implantation. Yet that assumption is unfounded. Recent studies justify the conclusion that Plan B does not produce a "hostile uterus." Instead, it delays ovulation and prevents conception.

The active ingredient in Plan B is levonorgestrel, a drug that has been used in birth control pills for 35 years. Taken on a daily basis, it causes changes in the uterine lining that are believed to make the uterus inhospitable to embryos. As a result, even the manufacturer of Plan B conceded it might work the same way when taken as an emergency contraceptive.

Over the past few years, however, scientists have learned that this is incorrect. When Plan B is taken as a one-day emergency contraceptive, it does not cause changes in the uterine lining. Instead, it acts by impeding the hormone surge that releases a woman's egg from her ovary.

The evidence comes from two independent lines of research. One consists of studies that have observed the changes produced in human female chemistry and reproductive anatomy when women take levonorgestrel. A thorough review of these studies by researchers from the Karolinska Institute in Sweden concluded that Plan B acts by delaying or blocking ovulation, probably by impeding the surge of a hormone that triggers it. It does not interfere with implantation.

The second avenue of research measured implantation rates directly. Researchers at the Catholic University of Chile designed two studies, one on rats and one on monkeys, to detect whether Plan B disrupted implantation. They found no evidence in either study that Plan B interfered with the implantation of fertilized embryos. When Plan B was administered after fertilization, pregnancy rates were the same for the females given Plan B as for those given a placebo.

Admittedly, this evidence is not conclusive. Scientists lack a test to determine whether a woman is carrying a fertilized egg. Thus, they are unable to study whether Plan B affects the fate of these eggs. That means conclusions must be based on the kinds of studies that were undertaken by faculty from Catholic University. They were looking for a credible answer to this question and were unable to detect any sign that Plan B interferes with embryo implantation.

This is remarkably good news. It means that neither pharmacists nor lawmakers need to face the choice between reproductive freedom and freedom of conscience. More important, women who have had unprotected sex no longer need to face the austere choice between an unintended pregnancy and an abortion. Instead, they can prevent conception.

Philip G. Peters Jr. teaches health law at University of Missouri-Columbia.

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