A team of international scientists has found a common genetic variant which may explain why some men with normal spermcounts and good quality sperm are affected by infertility.

The study findings suggested that men with a variation in a genewhich codes for a sperm-coating protein called beta defensin 126 (DEFB126) have a reduction in the protein coat on the outside of the sperm which makes it difficult for the sperm to 'swim' to theegg.

Dr Edward Hollox of the University of Leicester and co-author of the study said: 'If you've got this gene variant you should allow that little bit longer if your partner's planning to get pregnant'.

The researchers, including scientists from the University of California and the Anhui Medical University in China, carried out the study on over 500 newly-wed Chinese couples who were trying for a baby. They found that when men's sperm lacked a coat of the DEFB126 protein, their wives were significantly less likely than expected to become pregnant.

Previous studies have shown that two copies of the genetic variant may be found in up to one quarter of men around the world, with about half of all men having one copy. The DEFB126 protein coat helps sperm to swim through cervical mucus and evade the woman's immune system, as well as enabling it to attach to the walls of fallopian tubes.

The study showed, however, that men with two copies of thevariant produced sperm that were less able to swim through a substitute to cervical mucus, hyaluronic acid gel. In macaques, it has already been shown that this protein is important in evading the immune system and the researchers believe the protein coat plays the same role in humans.

Commenting on the study, Dr Allan Pacey, senior lecturer in Andrology at the University of Sheffield, said: 'We actually understand very little about the subtle molecular events which occur in sperm as they make their journey through the woman's body to fertilise an egg'.

The research was published in the journal Science Translational Medicine.