It has long been thought that biological clocks were strictly a female worry as men can father children even at an advanced age.
But research is increasingly confirming that men should not leave it too late to have children.
Just as many older men are slower than they were in their youth, researchers have found that embryos produced with the sperm of older men are slower to grow.
Another study, presented at the American Society for Reproductive Medicine in San Antonio, also found that sperm from older fathers accumulates more mutations.
IVF scientists in Boston found embryos created with sperm from men over the age of 50 divide more slowly and take longer to reach the early stage known as a blastocyst, when the embryo is ready to be implanted in the womb.
Researchers have found that embryos produced with the sperm of older men are slower to grow
For the study, timelapse video footage of 3,532 embryos taken from 527 couples was examined.
Those created with sperm from men over 50 developed 35 per cent more slowly than embryos created with sperm from men under 35.
Considering only embryos that reached the blastocyst stage, the time to reach this point was 4.3 per cent slower for the embryos of men over 50 than for the embryos of men under 35.
In a further study, researchers from Weill Cornell Medicine in New York showed that as men age, their sperm becomes more prone to develop mutations that result in DNA errors.
Sperm from older men was found to have an abnormal number of chromosomes in each cell – either too many or too few, a condition called aneuploidy. This will make the sperm less likely to create a healthy embryo.
The team analysed the DNA of sperm from men whose partners were experiencing recurring pregnancy loss.
The men were divided into seven age groups, with the youngest group aged 25 to 30 and the oldest over 55. Their sperm was tested for aneuploidy and DNA sequencing for a type of mutation called CNV or ‘copy number variation’.
Both types of defects were highest in the oldest age group. Fertilisation rate was 87.7 per cent in the youngest age group and declined to 46 per cent in the oldest group.
American Society for Reproductive Medicine vice-president Peter Schlegel said: ‘These studies make it clear that for men as well as women it may be difficult to have a child at an advanced age.
‘Older men and their partners should be counseled about the risks of poor pregnancy outcomes and the potential for neurodevelopmental problems in offspring.’