Emergency Contraceptives Prevent Fertilization - Is this News?

In the past few days, there has been significant coverage of a New York Times article about emergency contraceptives and whether or not there is evidence that they prevent the implantation of a fertilized egg. It is great that this issue is getting coverage but it is not new information!!

For years, scientists and public health advocates have been pointing to the preponderance of scientific evidence, which shows no post-fertilization mechanism of action for emergency contraceptives (ECs). In multiple studies, no evidence has been found to indicate ECs inhibit implantation.

Before fertilization can occur, sperm must move into the oviduct (the tube connecting the ovary and uterus) where they can stay for up to five days before dying. If an egg is released, it signals to sperm to swim towards the egg and 24 hours or less for the egg and sperm to unite. If fertilization occurs, implantation takes place about a week later but there are a lot of reasons that an egg naturally would not implant. As a result, women are encouraged to take EC as quickly as possible after intercourse before it is too late to prevent pregnancy.

So why do the labels of these drugs include a statement that they may work by blocking fertilized eggs from implanting in a woman’s uterus? According to the New York Times article, it stems from an FDA decision during the drug-approval process to mention the possibility on the label based on outdated guesses about how ECs work. At the time of its approval of Plan B (the most popular EC in the US), the FDA’s own description of an implantation effect was merely speculative, stating, "Plan B ‘could theoretically prevent pregnancy by interfering with a number of physiological processes” followed by a long list, including ovulation and implantation.’" The New York Times review of documents from the approval process documents found no evidence discussed which supported implantation effects. After reporters from the Times asked about the issue for their recent article, the firm that writes medical entries for the National Institute of Health website deleted sections suggesting that ECs could prevent implantation.

To make an informed choice, women must know that ECs (like other hormonal contraceptives) prevent pregnancy primarily by delaying or inhibiting ovulation and fertilization. In science, there are few definitive answers about how things work — instead, conclusions are made based on the preponderance of the evidence. In this case, the evidence shows that ECs prevent fertilization not implantation and women must know those correct facts about ECs, choose the method that is best for them, and take action as soon as possible after unprotected sex.