About 25% of all infertility is caused by a sperm defect and 40-50% of infertility cases have a sperm defect as the main cause, or a contributing cause.

It can be hard to know whether the sperm problem is the only cause, or just a contributing cause to the fertility problem. Part of the problem is that numbers are just numbers. We know that men with low sperm counts can sometimes have children - and some men with normal sperm counts can be infertile. If the sperm count or motility is extremely low, we usually assume this is the cause of the fertility issue. If the count or the motility is slightly low, it could be a contributing factor, but might not be the only fertility issue that the couple has. There could well be some female fertility problems as well.

What matters is not really how many sperm there are, or how fast the sperm swim - but whether they can fertilize the female partner's eggs. This is really a biochemical issue at the molecular level - looking at the little swimmers under the microscope is not a perfect way of assessing the ability of the sperm to fertilize the wife's eggs.