The sperms use subtle muscle contractions in the womb to arrive at their destination. And abnormal waves, or a complete absence of them, could account for nearly half of women with unexplained infertility failing to conceive, according to the latest issue of New Scientist magazine.
The waves - at their most intense in the middle of a woman's monthly cycle - are the result of fine contractions in the muscle layer of the uterus.
Edward Lyons, of the University of Manitoba Health Sciences Center in Winnipeg, Canada, says that the waves in healthy fertile women move upwards towards the Fallopian tubes 80 per cent of the time.
"Sperm are not salmon - they don't swim unaided upstream. They need these waves to ride on to get to the egg," he said.
He believes the force of the waves, as well as driving the sperm towards the Fallopian tubes, also keep them there.
A study found that in infertile women the waves were weak or infrequent, or moved downwards in the wrong direction towards the cervix.
"Abnormalities in the direction, frequency or intensity of these contractions may be an important cause of unexplained infertility," Mr Lyons added.