Going abroad for a relaxing holiday before trying for a baby may not be a good idea, fertility experts say.
Research has shown heat can harm sperm
Officials at the Bourn Hall Clinic in Cambridgeshire say soaking up the sun in tight trunks may be one of the worst preparations for fathering a child.
Australian research has shown that too much heat can reduce sperm count and slow down the surviving sperm.
But other experts cast doubt on the claims, although they said it may be wise to be cautious.
Many couples feel that a foreign break improves their chances of conceiving by allowing them to relax fully.
However, sperm takes eight weeks to mature in the testes, and during that period, is vulnerable to any changes in temperature.
The testes naturally hang outside the body to keep them cool, and confining them in tight underwear, or dipping them in the hot tub may not be a good idea, it is claimed.
Dr Thomas Mathews, the Medical Director at Bourn Hall, the world's first assisted conception clinic, said: "We find that at an initial consultation men may have a normal sperm count, but in some cases this has declined when we check again before treatment, which may be several months later.
"It is only recently that it has occurred to us that there may be a seasonal effect at work."
He said experiments had found that exposing the testes to heat over long periods led to a decreased sperm count.
In the 1950s, it was even suggested that regular hot baths could be a successful contraceptive.
Modern IVF methods do not require large numbers of viable sperm, as a single sperm can be used to fertilise an egg, in a process called ICSI.
The Australian research on animals found that heating the testes caused cell death, and a subsequent reduction in the numbers of sperm in the semen.
In addition, the sperm that were present had a reduced swimming ability, and might therefore be less capable of fertilisation.
But Sheffield University fertility expert Dr Allan Pacey said: "If this was a major effect, I think we would see people having problems conceiving in every country in the Tropics.
"However, it's worth being cautious, as there is some evidence that scrotal heating can have an effect on DNA quality in sperm."