A newly discovered cause of infertility could allow millions of infertile couples around the world to have a child. Researchers have found a common gene mutation that causes sperm to be far less able to swim through the mucus of the cervix and reach the egg.

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Normally, the gene causes the production of a protein called beta-Defensin 126 or DEFB126, which coats the surface of sperm. When the gene is mutated, the protein is missing, and the sperm have that problem in transit, according to researchers at the University of California, Davis, in a report published Wednesday. Wives of men with the genetic variation were less likely to become pregnant than other couples and 30 percent less likely to give birth, the researchers said.

About 15 percent of couples are infertile — a condition defined by the World Health Organization as the inability to achieve a pregnancy after a year of trying. In just under one third of the cases, the problem is with the man. Underdeveloped or abnormal sperm can cause male infertility, as well as hormone imbalances or certain medications. But many cases of unexplained infertility are likely connected to this genetic variation, the researchers suggested.

One reason this discovery is important is that the normal tests of the quality of sperm, such as examining them under a microscope and watching how quickly they move, do not detect the mutation. But with this new information a simple genetic test should soon allow doctors to find it easily. For couples who have the genetic mutation doctors can use insemination to get the sperm right next to the eggs and overcome the handicap. If that relatively simple intervention doesn't work, they can resort in vitro fertilization, even injecting a single sperm into the egg, if necessary.

It appears that about one in every 250 men has this genetic mutation.

The work was carried out by doctors and scientists from UC Davis; Simon Fraser University in Burnaby, BC, Canada; the University of Leicester I, UK; Anhui Medical University in Hefei, China; and the University of Illinois, Chicago. It was published in the journal Science Translational Medicine.