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Boys born to fathers who needed help conceiving have poorer sperm quality as adults than peers conceived without help, a study suggests.

The study, in Human Reproduction, looked at men conceived using Intra-cytoplasmic sperm injection (ICSI).

The researchers said it confirmed the theory boys would inherit poor fertility from their fathers.

But a UK expert said the study was "reassuring" because the sons' results were not exactly like their fathers'.

In ICSI, a single, good quality sperm is selected and injected directly into an egg.

The technique was developed in the early 1990s to help men with a low sperm count, abnormally shaped sperm or sperm that does not move well.

In 2013, 37,566 embryos were transferred in the UK using ICSI, just over half of all IVF treatments.

'Poor relationship'

This study, carried out by a team from the Universiteit Brussels - where ICSI was developed - looked at 54 men aged 18 to 22. They were compared with 57 men of the same age.

Men born from ICSI had almost half the sperm concentration and a two-fold lower total sperm count and motile sperm - that can swim well - than men of a similar age whose parents conceived naturally.

They were also nearly three times more likely to have sperm concentrations below the World Health Organization's definition of a "normal" level - 15 million per millilitre of semen - and four times more likely to have total sperm counts below 39 million.

Prof Andre Van Steirteghem, who led the study, said it had been the first chance to test the long-held theory that sons would inherit sperm quality issues.

But he said it was a more complex picture than might have been expected: "Semen characteristics of ICSI fathers do not predict semen values in their sons.

"It is well established that genetic factors play a role in male infertility, but many other factors may also interfere."

Prof Richard Sharpe, leader of the Male Reproductive Health Research Team at the University of Edinburgh, said that since most cases of male infertility were unexplained, it was uncertain that the father's fertility problems would be "inherited".

He added: "Importantly, the results are a reminder to us that ICSI is not a treatment for male infertility, but simply a way of bypassing a problem and leaving it for the next generation to deal with - something my generation seem horribly adept at doing."

But Allan Pacey, professor of andrology at the University of Sheffield, said the findings were "reassuring".

"Twenty years ago we were telling parents that their sons might have the same problems as they did and that they would also need ICSI to reproduce. But this suggests that might not always be the case."