By Martin Hutchinson
BBC News Online health staff in Madrid
Men have traditionally been told to refrain from having sex before certain types of fertility treatment - but research might change that.
Sperm cluster around an egg
In fact, long periods of abstinence might actually reduce sperm quality, it is claimed - couples might be told to have sex a few days before coming to the clinic.
Many are told not to have intercourse for at least a week before their semen is collected.
The World Health Organisation guidance to fertility clinics recommends abstinence of between two and seven days.
For these patients we recommend minimal abstinence - ideally no more than two days
Dr Eliahu Levitas, Soroka University
However, a team of researchers from Soroka Univerisity and Ben-Gurion University of the Negev, in Beer-Sheva, tested thousands of semen samples.
They looked at not only the overall quantity produced, but also the sperm concentration and shape - and the percentage of sperm that were moving properly.
They found that while the volume of semen was greater after between 11 to 14 days abstinence, other markers of quality deteriorated.
The shape and form of the sperm gradually got worse - in the samples taken from men with reduced sperm counts, the proportion of moving sperm dropped significantly from day two onwards.
Dr Eliahu Levitas, who led the research team, is presenting his work at the European Society of Human Reproduction and Embryology annual meeting in Madrid on Monday.
He said: "Our data challenge the role of abstinence in male infertility treatments.
"What we have found is not so relevant to ICSI, where a single sperm is injected into the egg, but for those treatments where we are trying to get the best possible sperm quality for intra-uterine insemination.
"For these patients we recommend minimal abstinence - ideally no more than two days."
There is no evidence suggesting why sperm might become less viable if it spends longer in the testes.
Dr Levitas said: "It is possible there is oxidative DNA damage from, for example, cigarette smoking or drinking."