There was a time when he was as well known for his tight trousers as he was for his peroxide blonde hair and gravelly voice. But these days Rod Stewart is more likely to be wearing something a little looser below the waist.

The reason, his wife Penny Lancaster explained this week, is that Stewart, now 70, was advised by a doctor to stop wearing such constrictive clothing while the couple were trying to conceive a second child.

Rod Stewart in 1982

Speaking on ITV's Loose Women, Lancaster, 44, said, "When Rod and I were trying to make our second child, we were having a few troubles and a doctor said, 'Rod, you've got to forget wearing the tight pants and trousers, OK? Get rid of the Calvin Kleins, do the loose boxer shorts, and no hot baths.'"

Lancaster gave birth to the couple's second child (and Stewart's eighth), Aiden, in 2011.

Last month, Lancaster caused controversy by appearing to suggest that men should stay out of the kitchen because cooking "belittles men and takes away their masculinity". On Loose Women, she explained that she had been trying to draw attention to concerns over the fertility of men who spend a lot of time standing in front of a hot oven.

Temperature is widely believed to have a significant effect on male fertility. The optimum temperature for sperm production is 34.5 degrees celsius, which is slightly below body temperature. A three-year University of California study in 2007 found that five out of 11 men who stopped taking hot baths experienced a sperm count rise of almost 500 per cent.

Penny Lancaster and Rod Stewart in March 2015

^{(Photo: PA)}

Earlier this year, Paul Serhal, medical director at the London Centre for Reproductive and Genetic Health, advised men who are looking to conceive to avoid wearing very tight clothing, such as skinny jeans.

"Skinny jeans might be fashionable but they’re not conducive to fertility," he said. "If the testicles are squashed, this will generate friction which will in turn generate heat."