IVF study: Conception is a rare event

A review of all the efforts to unite egg and sperm at Shady Grove Fertility Centre in Maryland showed just 7.5% of tries resulted in an embryo that could then be implanted in a woman's womb, a team led by Michael John Tucker said.

"It should surprise no one that the vast majority of sperm and eggs never get together to even begin the fertilization process," Dr Robert Rebar, executive director of the American Society for Reproductive Medicine, said in a statement.

"But, it is very important to understand that even once joined together for fertilization, an overwhelming majority of fertilized eggs do not become viable embryos, and only a small percentage of embryos thought to be viable produce a child. While this data come from the IVF lab, natural conception is also very inefficient," Rebar added.

They reviewed all the in vitro fertilization or IVF cycles at their centre between 2004 and 2008. Out of 110,000 egg cells fertilized with sperm, only 31,437 resulted in viable embryos.

Usually just one or two embryos are implanted at a time, and the others are frozen.

But assuming that all the frozen embryos would eventually be used, 8,366 babies would theoretically be born - just 7.5% of all the fertilized eggs, the researchers said.

Earlier this month the Nobel prize for medicine or physiology went to British physiologist Robert Edwards, whose work led to the birth of the first "test-tube baby," Louise Brown, in 1979.

As many as 4 million IVF babies have been born since then.

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