We tend to think of women’s declining fertility as being the reason older couples can struggle to conceive. But, in fact, the growing number of men putting off fatherhood until later in life may find there are big consequences for them and their kids – or lack of them.

One scientist, Dr Kevin Smith of Dundee University, has even gone so far as to call for the NHS to start freezing the sperm of all 18-year-old men in the UK. He argues this could enable the men to have children later in life without some of the risks associated with doing so.

The “right time” to become a parent is less clear biologically for men than for women. Men don’t go through an equivalent to the menopause. But this may have lured us into a false sense that fatherhood is just as natural at 70 as at 20 years old.

The social acceptability of older fatherhood has been reinforced by celebrities such as Paul McCartney, Rod Stewart and Steve Martin becoming dads again in their middle-to-old age. Society assumes that sperm from older men are just as healthy as those from young men. But the evidence suggests this is a myth.

Declining quality

Most scientists agree that the quality, if not the quantity, of sperm has decreased in the past 50 years. “Just 25% of European men have good-quality sperm” we were told by the University of Copenhagen’s Niels Jorgensen at a recent conference, where he reported his 70 years' worth of data. This decline in quality, he said, can lead to longer times to natural conception, smaller families and as many as “15% of men [needing] to resort to fertility treatment to have a family”.

However, given that a man can produce up to 500m sperm in a single ejaculate, the winning sperm is likely to be among that man’s best, whether successful in spontaneous conception or with fertility treatment. So the risk of that particular sperm being of poor quality and causing problems with resulting children isn’t likely to be significantly affected by declining overall quality. The latest reassuring evidence suggests that artificially conceived children achieve equal academic performance and experience no increased risk of cancer compared to children conceived naturally.

No time to waitShutterstock

Because this decline in the male population’s semen quality has been observed over such a short time interval, it may be caused by lifestyle, occupation and environment rather than by genetic anomalies. If so, we may be able to prevent any further impairment or even improve semen quality again (although it won’t be easy).

However, the growing number of older fathers is more concerning. There is much evidence that sperm quality falls as men get older, from around the age of 40. Unlike the problems with young men’s sperm quality, problems for older men are more likely to be genetic, less likely to be reversible and come with possible short and long-term effects on potential children.

Men matter

The impact of how men’s DNA changes over time on the health of any children they father is a much-neglected topic. Even in the 21st century, we tend to consider “mother and baby” health together and reduce the father to a single sperm of standard quality. This is tantamount to treating human reproduction as an asexual process where the mother effectively clones herself (parthenogenesis).

Over the past decade the rapid advances in molecular technology have shown that assumption to be incorrect. Numerous studies report links between older fathers and increased risk of genetic diseases specifically associated with DNA mutations, abnormal numbers of chromosomes, and the effect of environmental (epigenetic) factors on genes.

Together, these robust studies show associations between older fatherhood and greater rates of cancer, childhood mortality and mental disorders such as bipolar disease, autism and schizophrenia.

One piece of research suggests that children born to fathers older than 45 are more likely to die in childhood than the general population. Similar evidence was found showing a greater likelihood of suicide attempts, substance abuse problems and low educational attainment.

Smith has done this issue a great service by raising the perils of delayed fatherhood. While I do not believe the risks justify freezing the sperm of every 18-year-old in the country – nor do I think men would use this service – Smith’s suggestion has raised the profile of this problem a lot in just one day.

Is there an age when men are less “fit for fatherhood”? We certainly have sufficient evidence to suggest that healthy babies are more likely to be born to young men as well as young women. When it comes to parenthood, men need to “man up” sooner.

Sheena Lewis receives funding from NIHR and EU. She is also CEO of a university spin out company , Lewis Fertility Testing Ltd