IVF success rates decline significantly when they involve men aged over 51, a study has suggested.
Scientists said their findings warrant a call for a public message to men to heed their biological clock and not to delay fatherhood.
The analysis of 5,000 IVF cycles performed at a centre in London showed clinical pregnancy rates declined with increasing paternal age, researchers said.
The rates in men under 35 were at almost 50 per cent while this dropped to 30.5 per cent in those aged over 51.
When re-analysed in a statistical model which included maternal age, scientists found that, for all maternal age subgroups, the probability of pregnancy still fell significantly when the man was aged over 51.
Dr Guy Morris from the Centre for Reproductive and Genetic Health (CRGH) in London, who was behind the research, said that while previous studies have demonstrated a decline in natural male fertility, it was not known whether paternal age affected outcomes in IVF.
He added that while some clinics and health authorities set an upper age limit on female IVF patients, no such limits are known to exist for males.
The results of the study were due to be presented at the European Society of Human Reproduction and Embryology's annual meeting on Wednesday in Vienna.
Dr Morris said: "There may well be a public perception that male fertility is independent of age. Stories of celebrity men fathering children into their 60s may give a skewed perspective on the potential risks of delaying fatherhood.
"Indeed, in natural conception and pregnancy it is only recently that evidence of risks associated with later fatherhood has become available.
"These more recent studies contrast with decades of evidence of the impact that maternal age has on fertility outcomes.
"In the context of this emerging evidence for the deleterious effect of increasing paternal age, our data certainly support the importance of educating men about their fertility and the risks of delaying fatherhood."
