The analysis, of nearly 5000 IVF/ICSI cycles, suggested that this was the case regardless of the woman's age.
'There may well be a public perception that male fertility is independent of age,' said Dr Guy Morris from the Centre for Reproductive and Genetic Health (CRGH) in London, where the retrospective study was carried out. 'Stories of celebrity men fathering children into their 60s may give a skewed perspective on the potential risks of delaying fatherhood.'
He presented the study at the annual meeting of the European Society for Human Reproduction and Embryology in Vienna, Austria.
The research team from CRGH and University College London analysed the records of 4271 men, in couples which underwent 4833 cycles of IVF and ICSI treatment between 2009 and 2018. These couples were being treated for all causes of infertility.
The study showed that only 42 percent of men over 51 met the World Health Organisation's standard semen references values, compared with 61 percent of men under this age.
While the pregnancy rate decreased with age for women over 35, this was also true – regardless of maternal age – with increasing paternal age.
Experts were unsurprised by the result. 'Men's sperm quality decreases with age,' said Professor Sheena Lewis from Queen's University Belfast and Examenlab. 'This is why all donor sperm banks have an upper age limit – usually 45 years.'
She added: 'We must dispel the public myth that men can father children at any age. It may take longer and childhood health may be affected.'
'This study again highlights that men are not immune to the impact of biological ageing as far as their fertility is concerned, although this detrimental impact arrives later than women,' said Professor Ying Cheong at the University of Southampton.
'Our data certainly support the importance of educating men about their fertility and the risks of delaying fatherhood,' said Dr Morris.