Poor sperm could be to blame for women who experience recurrent miscarriages, a study has suggested.
Doctors generally look to the mother for the cause of a miscarriage, but scientists believe the father's health also plays an important role.
The sperm of men whose partners have three or more miscarriages tend to have more DNA damage, researchers have found.
Experts say this could trigger the failed pregnancies and further trials are needed to confirm the findings, in hoping of paving the way for new fertility treatments.
The researchers now say working out why men's sperm are becoming damaged is also important.
They suggest past infections or STIs, obesity, or being older could trigger higher levels of DNA-damaging molecules.
Research by Imperial College London found the partners of women who suffered recurrent miscarriages have twice as much DNA damage as the average man.
And they have four times higher levels of oxygen reactive species, common but potentially damaging molecules which the scientists suggest could corrupt DNA.
'Traditionally doctors have focused attention on women when looking for the causes of recurrent miscarriage,' said lead researcher Dr Channa Jayasena.
'The men’s health – and the health of their sperm, wasn’t analysed.
'However, this research adds to a growing body of evidence that suggests sperm health dictates the health of a pregnancy.
'For instance, previous research suggests sperm has an important role in the formation of the placenta, which is crucial for oxygen and nutrient supply to the foetus.'
Recurrent miscarriages – the loss of three or more pregnancies before 20 weeks – affect around one in 50 couples in the UK.
Infection or immune problems in the mother have traditionally been viewed as possible causes.
But Imperial's small study of 110 male volunteers revealed significant differences in the sperm of men whose partners suffered miscarriages and those whose didn't. They took sperm from 50 men whose partners had suffered repeated miscarriages and compared it to samples from 60 other men.
The oxygen reactive species, of which the miscarriage-affected men have more, protect sperm from bacteria and infection but can be harmful in high concentrations.
Researchers now want to work out what affects men's levels of these molecules, suggesting it could be a history of STIs, obesity or age.
Dr Jayasena said: 'Although none of the men in the trial had any ongoing infection such as chlamydia – which we know can affect sperm health – it is possible there may be other bacteria from previous infections lingering in the prostate gland, which makes semen.
'This may lead to permanently high levels of reactive oxygen species.'
He also explained high levels of body fat are also believed to trigger an increase in the molecules, and researchers found men in the miscarriage group were heavier.
Dr Jayasena added: 'Although this is a small study, it gives us clues to follow.
'If we confirm that high levels of reactive oxygen species in semen increase the risk of miscarriage we could try to develop treatments that lower these levels and increase the chance of a healthy pregnancy.
'It has taken medicine a long time to realise sperm health has a role to play in miscarriage – and that the cause doesn’t lie solely with women.
'Now we realise both partners contribute to recurrent miscarriage, we can hopefully get a clearer picture of the problem and start to look for ways of ensuring more pregnancies result in a healthy baby.'
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