Men have a better chance of having kids if they have semen teeming with strong-swimmers, but fertility experts now claim that sperm which had tails of a similar length were better able to travel than those with tails of varying lengths.

The team led by James Mossmon were surprised to find that tail consistency trumped average length, the Daily Mail reported.

The findings add to our understanding of why some couples struggle to have children.

Around one in six couples may have difficulty conceiving in the UK - which is around 3.5million people.

Couples are recommended to visit their GP if they have not conceived after one year of trying, or sooner if the woman is aged over 35.

For the study, researchers from Brown University in Rhode Island examined the semen of 103 men attending an infertility clinic at Sheffield Teaching Hospital NHS Foundation Trust.

They found that the wider the variation of sperm length in samples, the lower the concentration of motile sperm. The variation in tail length was found to be the most crucial factor.

"Sperm length measurements may provide a useful insight into testis function and the efficiency of spermatogenesis (sperm cell development)," the scientists said.

It is another piece in the jigsaw that explains why only one percent of the 300million sperm released by a man during sex manages to reach their partner's uterus, while just a few dozen reach the egg.

The findings are published in the journal Human Reproduction.