According to Dr Silber, of St Luke's Infertility Hospital in the US, until recently we have had a poor understanding of the effect of ageing on male fertility. As many examples have been offered of older men having babies, the thought of decreased fertility in men was never greatly addressed. "It was assumed that male fertility was relatively immortal because so many elderly men have been able to impregnate their wives," says Dr Silber. "However, there has been previous crude data showing a relative decrease in sperm count, and possibly fertility, in a certain percentage of ageing men. Now, the field of male fertility has come to be mainstream and full of new research, new data and new conclusions."

Research is exploring many of the whys and how’s of decreased sperm production and maturity with age. As a result, many explanations have been uncovered.

"Men experience an age-related decrease in testicular size and sperm production," says Dr Silber. "In some men, there is a decline in testosterone production that becomes noticeable after the age of 40. The loss of testosterone can result in a decrease in bone and muscle mass in the ageing male, the loss of sex drive, the decreased ability of the body to produce and mature sperm cells, as well as the inability to obtain or maintain an erection. Both the decrease in testosterone and the decrease in sperm production cause an age-related decrease in fertility. Also, sperm may also be affected by repeated ejaculation decreasing the secretions of the glands, the decrease of the number of hormones and the weakening of the sexual muscles."

Some of the most common effects of age on the sperm that is produced include the following:

• Decreased motility: Sperm that has not matured will not have the adequate motility to reach and penetrate the egg. Also, with age comes a decreased ability to have strong ejaculations, thus, decreasing the distance that the sperm will travel upon ejaculation.
• Decreased strength: Immature sperm will not have the needed strength to travel the distance to the egg, nor to penetrate the membrane for fertilisation.
• Decreased potency: The force of the ejaculatory burst in young men is often powerful and can eject the sperm some distance. The force of the squirt, propelled by the powerful contraction of the bulbocavernosus muscles, is much less in older men than in younger men. Thus, in every measurable way male potency is affected by age.
• Altered genetic make-Up: As men age, sperm cells can accumulate mutations that are passed to offspring. Regardless of age, sperm continues to reproduce through division. If a sperm becomes altered or mutates, any other sperm that is produced by the natural division will also be altered or mutated. Each successive division introduces a slight risk of error in the genetic material of the new sperm, which is passed on to the children.

"Sperm studies in ageing men often show kinetic or formation disorders," says Dr W-B Schill, professor in the Department of Dermatology and Andrology at Justus Liebig University in Giessen, Germany. "Most observed were impaired spermatogoniogenesis (reproduction, dividing and development of the sperm cell) or spermatid (a maturing sperm cell) malformations, paralleled by a highly significant decrease in daily sperm production. This indicates and accompanies a gradual decline of fertility with increasing age. And, although alterations in sperm quality may be apparent, reduced motility and a lower percentage of spermatozoa (a mature sperm cell) with normal morphology are most frequent."

This is why, following HFEA regulations, we accept sperm donors only between the ages of 18-41. This age range appears to allow the best chance of producing healthy sperm that will be able to create a viable pregnancy.