Boys born through a common type of IVF treatment are suffering from low sperm counts and may not be able to have children naturally, a study suggests.
Doctors in Brussels pioneered intracytoplasmic sperm injection in the 1990s and have been following 54 children born using the procedure ever since.
The technique, developed to help male infertility, involves selecting a healthy sperm and injecting it into an egg before the resulting embryo is implanted into the mother. It is now commonly used in fertility procedures even when the woman is infertile, because selecting the best sperm increases the chance of a healthy baby.
However, experts were concerned at the time that baby boys born from this technique would also inherit their father's infertility, meaning future generations would always require ICSI to become parents.
In the first study of its kind, the team who carried out the original procedure discovered that the 54 men, who are now aged between 18 and 22, had almost half the sperm concentration of naturally conceived men as well as a 62 per cent lower sperm count, and 66 per cent lower sperm motility - which measures how well a sperm can move.
The men were also nearly four times more likely to have sperm counts below the level deemed normal by the World Health Organisation.
Prof Andre Van Steirteghem who headed the Centre for Reproductive Medicine at the Vrije Universiteit Brussel in Belgium, where the technique was developed, said: "These findings are not unexpected.
"Before ICSI was carried out, prospective parents were informed that it may well be that their sons may have impaired sperm and semen like their fathers. For all the parents this information was not a reason to abstain from ICSI because, as they said: 'if this happens ICSI can then also be a solution for our sons'.
"These first results from the oldest group of ICSI-conceived adults worldwide indicate that a degree of 'sub-fertility' has, indeed, been passed on to sons of fathers who underwent ICSI because of impaired semen characteristics."
The first ICSI baby was born on 14 January 1992 and all the men in the study were born in the early years of the technique, between 1992 and 1994.
The researchers adjusted their results for factors that could affect semen quality, such as age, body mass index and sexual malformations.
Around half of the 60,000 annual fertility treatments in Britain involve ICSI and it is often used when the cause of infertility is unknown.
However, the authors warn that the findings from this study cannot be extrapolated to all offspring born after ICSI since it can be used when there is no evidence that a couple's infertility is due to abnormal sperm.
Allan Pacey, professor of Andrology at the University of Sheffield, said: "When the ICSI procedure was first introduced into clinical practice there was much discussion and debate about what the future fertility prospects would be of any males that were conceived this way, when they reached adulthood.
"This was because the reason for using the ICSI procedure to conceive them in the first place was because of the father's poor sperm quality, which was almost certainly of genetic origin.
"Therefore, if those genes were inherited by any son's born through the ICSI procedure, it seemed likely that they would have the same level of infertility and potentially require the use of ICSI themselves when they wanted to become a father."
Prof Simon Fishel, managing director of CARE Fertility, added: "We may still find that these men are able to conceive naturally. Just having low semen parameters is not evidence for the requirement of ICSI or IVF technologies. We know many men with such are indeed able to conceive naturally.
"More follow up studies will be required to ascertain meaningful outcomes." The research was published in the journal Human Reproduction.