Men who wear skintight cycling shorts could be harming their chances of getting their partner pregnant, experts have warned.

So-called mamils – or middle-aged men in Lycra – should consider looser clothing when trying for a baby, they say.

Isabel Traynor, a lead nurse for assisted conception at NHS Greater Glasgow and Clyde, said tight shorts can affect sperm counts and quality by excessively heating the testicles.

She raised the issue at the Progress Educational Trust fertility event after discussions with the female partner of a cyclist.

‘We advise them to wear loose underwear and avoid anything tight around their testes when exercising as this may have an insulating effect, increasing temperature, which potentially affects sperm counts and quality,’ she said after the event.

While tight-fitting bike wear is popular among male cyclists, it has its detractors. Six-times Olympic track cycling champion Sir Chris Hoy wrote in GQ magazine last year that Lycra ‘generally looks awful on pretty much anyone heavier than eight stone’.

The testicles need to be several degrees cooler than the rest of the body to produce sperm effectively and studies show a rise in temperature can change its shape, speed and count. Male fertility expert Allan Pacey, of the University of Sheffield, said: ‘I am not aware of any evidence that mamils are a risk of having poor sperm quality.

‘But we do know from several studies, including some of mine, that men who wear tight underwear generally have poorer sperm quality than men who wear loose ones.

‘I suspect wearing tight shorts might have the same effect – heating up the testicles – but that is an assumption at the moment.’

Experts do not advise men to stop cycling, as sperm quality can improve with moderate exercise. ‘Therefore, perhaps its just a case of wearing looser sports clothing,’ said Professor Pacey.

Studies have also suggested tight jeans increase the risk of lower sperm quality.

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