For centuries men have reassured themselves that in an infertile partnership it is nearly always the result of a problem with the woman’s reproductive system.

Men believe that if they had a low sperm count, if the sperm didn’t swim strongly in the right direction or if too many were of abnormal appearance, it could be that their sperm or lack of them was a factor.

Men never thought that even if their sperm count was normal and they were of reasonable appearance and agility that they could still be unfit for purpose. Modern molecular medicine has shattered this male complacency. The genetic integrity of the sperm is also important if they are to be efficient at fertilising the ovum, and the subsequent development of the embryo is to be normal.

For a year or two it has become accepted that evidence of DNA fragmentation in sperm cells may represent one reason why a couple are infertile, even though every other aspect of their sperm count and examination is normal.

For the past couple of years men attending my practice, who were in an infertile partnership, have had the quality of the DNA in their sperm cells assessed and the amount of fragmentation determined. This should now be done routinely if there is unexplained infertility, arrested embryo development after conception, recurrent miscarriages, if the man is older than average or has a varicocoele - varicose veins of the scrotum that may keep testicular temperature too high for healthy sperm development.

Apart from these causes of DNA fragmentation it has also been found that this is more likely if a man has an illness accompanied by a fever, a genital infection, is too fat, smokes too much, drinks too much or if he takes recreational drugs such as cannabis. Recent research proving the link between infections with chlamydia or mycoplasma and increased sperm DNA fragmentation answers many of the questions raised by clinical observations from the late 1970s onwards.

The late Dr Eric Dunlop of the Royal London Hospital demonstrated that NSU, nongonococcal inflammation of the urethra in the male, and the subsequent pelvic inflammatory disease in the woman reduced fertility. He also showed that large doses of doxycycline given to both partners for twice the usual length of time frequently resulted in conception.

It was always assumed that chlamydia usually caused infertility because the woman’s tubes had become inflamed and obstructed. Occasionally it was accepted that a man might have reduced fertility because he had a similar problem and was suffering from epididymitis affecting his tubes, but no one suspected that the integrity of his sperm DNA was important.

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