Men who want to become a father should wear boxer shorts rather than briefs – and go to bed in the nude.
Researchers have found that a man’s choice of underwear makes a big difference to the quality of their sperm.
It seems that airier, cooler underpants help preserve vital reproductive cells.
And at night, wearing no underwear at all gives the best results.
Scientists from Stanford University in California and the US National Institute of Child Health and Human Development carried out tests on 501 men who were trying to become fathers.
Of the group, those who wore loose boxer shorts during the day and no underwear in bed had 25 per cent lower levels of damaged DNA in their sperm compared to those who wore tight pants both at night and during the day.
The researchers, who will present their findings at this week’s meeting of the American Society for Reproductive Medicine in Baltimore, suspect that heat is the reason for the effect.
The scientists, led by Katherine Sapra of the NICHD, will say: ‘Among men in the general population attempting pregnancy, type of underwear worn during the day and to bed is associated with semen quality.
‘Reducing exposure for bed decreases DNA fragmentation; better semen quality parameters are observed in men wearing boxers during the day and none to bed.’
Previous research has also found that men who sit with laptops on their knees for prolonged periods are more likely to have damaged sperm, an impact also thought to be linked to heat.
Male fertility expert Allan Pacey, professor of andrology at Sheffield University, said that tight underwear is bad for sperm production – and encouraged prospective dads to choose loose undergarments.
‘We have known for some time that men who increase the temperature of their testicles, either through the heat exposure at work or by wearing tight underwear, have poorer semen quality compared to men whose testicles are cooler,’ he said.
Although there is a clear link between underwear and sperm, Professor Pacey said that as yet there is no definitive proof that it affects birth rate.
‘What has never adequately been shown is whether men can improve things by changing the choice of their underwear, but this study - although quite small - goes some way to suggest that is true,’ Professor Pacey said.
‘What we really need to see is whether switching to looser underwear makes their partners get pregnant more often or quicker than they would have done had the men continued to wear tight underwear.
‘Having said that, switching underwear is hardly a risky thing to do, so buying some looser pants might be good advice for would be fathers.’