Fewer than eight percent of all tries at making a baby in a lab dish will succeed, U.S. researchers reported on Monday.
A review of all their efforts to unite egg and sperm at their fertility clinic showed just 7.5 percent of tries resulted in an embryo that could then be implanted in a woman's womb, a team led by Michael John Tucker at Shady Grove Fertility Center in Maryland 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.
The Shady Grove team will present their findings to an ASRM meeting in Denver this week.
They reviewed all the in vitro fertilization or IVF cycles at their center 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 percent 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.
A second study to be presented at the meeting sheds light on another aspect of infertility.
Jorge Chavarro and colleagues at Harvard Medical School and Massachusetts General Hospital found that the more fat a man ate, the lower his sperm concentration.
They studied 91 men seeking fertility treatment.
Men ranking in the top third for saturated fat intake had 41 percent fewer sperm than men in the lowest third, they reported. Men in the highest third for monounsaturated fat intake had 46 percent fewer sperm than men in the lowest third.
"Men who are planning to father children should be encouraged to achieve and maintain a healthy weight and to consider their diets. What you eat affects your whole body, including your sperm cells," said Nancy Brackett, president of the Society for Male Reproduction and Urology.