Vasectomies can cause chromosomal abnormalities in sperm, Thai researchers have shown.
By Caroline Ryan
BBC News, Prague
They found men who had vasectomy reversals had 10 times the rate of abnormalities of normal men's sperm.
But the team told a European fertility conference it was the original operation, rather than the reversal, that caused the faults.
A UK expert backed the researchers' view that men should be able to freeze healthy sperm ahead of a vasectomy.
About 40,000 vasectomies are carried out in the UK every year, with 2,500 having the procedure reversed - half of which are successful.
Professor Nares Sukchareon, of the Department of Obstetrics and Gynaecology, Chulalongkorn University, Bangkok, studied men with a condition where their sperm cannot mix with ejaculatory fluid.
It was found they had much higher rates of chromosomal aneuploidies - abnormalities.
Improved with time
They also saw some suggestion that the problem resolved over time - so the longer it was since a man had had his vasectomy reversed, the more likely he was to produce normal sperm.
If couples conceive naturally the presence of some faulty sperm should not be a problem as the sophisticated female reproductive system selects the best.
But Professor Sukchareon said IVF doctors needed to be aware of the potential for higher miscarriage rates, and monitor carefully children born as a result, particularly if ICSI or other forms of assisted reproduction which specific sperm was involved.
He added: "This study raises a lot of questions. Is the abnormal sperm reversible, and if so, how long will it take before things get back to normal?
"Perhaps more importantly, will babies born after vasectomy reversal - either conceived naturally or by IVF - have problems themselves?"
So far, no studies have been carried out to look at this, he said.
He added: "I would recommend men freeze their sperm in case they change their mind about their vasectomy."
Dr Allan Pacey, Honorary Secretary of the British Fertility Society, said the study findings were worrying.
He said scientists had always been aware there was the potential for damage in vasectomies, but that the technology to examine sperm had only recently been available.
He added: "I think freezing sperm would be a good idea. I have had a number of men who say this wish it was available.
"There are people who believe that would not show appropriate commitment to the vasectomy procedure.
"But I think it would be a sensible insurance policy."