Men with low sperm counts may have an increased risk of passing on genetic conditions to children conceived using fertility treatment, researchers warn.
Researchers examined the genetic make-up of sperm
Scientists from the University of Porto in Portugal compared sperm DNA from men with normal and low sperm counts.
The lower the sperm count, the more genetic defects were observed.
Writing in The Lancet, they say these faults could increase the risk of the developing embryo being affected by certain genetic conditions.
Concerns have been raised that children conceived through IVF are at an increased risk of genetic imprinting disorders.
These include Beckwith-Wiedemann syndrome, which causes too much growth and is linked with an increased chance of tumours, and Angelman's syndrome, which affects the development of the brain.
It had been thought this risk could be linked to the actual process of IVF itself.
But this study looked at whether the increased risk could be due to the genetic make-up of the sperm.
The researchers, from the Institute of Biomedical Sciences at the University of Porto took sperm DNA from 123 men with low or normal sperm counts.
They examined specific imprinting genes which, if they do not behave in whichever way they should, can cause faults in other genes.
These are key genes where either the maternal, paternal or both versions need to be "switched on" for them to work as they should.
If they are not expressed correctly, the genetic malfunctions are then likely to be passed onto the embryo, potentially affecting its development and causing the child to be affected by a genetic disorder.
The researchers found that in all 27 men who had normal sperm counts, the imprinting genes behaved in the correct way.
But faults were observed in just under one in four of the 96 men with low sperm count who were studied.
Professor Mario Sousa said the study had shown men with low sperm counts carried an risk of transmitting imprinting errors to children conceived through IVF.
The study did not look at children born to men with low sperm counts.
Professor Chris Barratt, Head of Reproductive Medicine at the University of Birmingham, told BBC News Online: "These conditions are very rare in children.
"But the incidence of genetic faults in sperm has been shown by this study to be higher in men with low sperm count - therefore it is a significant issue."
Professor Barratt said couples undergoing fertility treatment because the man had a low sperm count should be counselled about the risk of their child being affected by a genetic condition.
"What the patient then does with that information is up to them. They have to decide whether to take the risk of not have a child."
He said scientists could carry out tests on sperm to see if men were affected, but this would only be possible in research centres due to their complexity.
But he added: "This is a further concern regarding assisted conception."