It prevents ovulation or fertilization of an egg.

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Because egg freezing requires the use of in vitro fertilization for future pregnancies, its widespread adoption would also likely increase the use of preimplantation sex selection and genetic diagnosis, practices which pose a range of ethical dilemmas.

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If there has been fertilization, Plan B may prevent a fertilized egg from embedding in the uterus.

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In 1968, Dr. Edwards teamed with Dr. Steptoe to study fertilization and develop methods that would allow the sperm and egg to survive outside the womb.

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Even more radical was the invention, in 1978, of in-vitro fertilization, or IVF, the now-common technique in which egg and sperm meet in a laboratory petri dish and create an embryo that is then implanted in the would-be mother's womb.

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Because it is the age of the eggs not the uterus that most affects the success of conception, egg freezing allows women to preserve younger eggs and then use them later through in vitro fertilization.

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There is another benefit of egg-freezing technology that many people don't know about: If your own frozen eggs don't survive thawing or fertilization or fail to grow into robust embryos, you can go online to one of the many commercial egg banks popping up across the country and order a batch of frozen eggs donated by a woman who looks like you.

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