We all know it takes two to tango, but even though males contribute half the genetics of a pregnancy, they are too often left out of the picture. This is especially true when it comes to studies on miscarriage and recurrent pregnancy loss (RPL).

A growing body of research is slowly revealing what an oversight that truly is. Because even though carrying a baby to term is commonly thought of as a female issue, that is almost certainly a misconception.

In reality, 60 percent of miscarriages are caused by a genetic problem, and that suggests male sperm is also partly responsible.

Investigating the sperm quality of 50 males whose partners had experienced three or more consecutive miscarriages, a new study offers even more evidence that poor quality sperm may be an important risk factor for RPL.

Compared to 60 males whose partners had not experienced any miscarriages at all, the males involved in this study showed twice as much DNA damage in their sperm.

"Traditionally doctors have focused attention on women when looking for the causes of recurrent miscarriage. The men's health - and the health of their sperm, wasn't analysed," says lead author Channa Jayasena, an expert in reproductive endocrinology and andrology at Imperial College.

"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."

Believe it or not, when it comes to RPL, Jayasena's study is the first research to evaluate the hormonal and metabolic health of sperm. And while the sample size investigated may be small, the findings have given researchers several important clues to follow.

The authors are now proposing that sperm damage may be triggered by a reactive oxygen species (ROS), found in the fluid that bathes sperm.

These ROS molecules are typically beneficial, protecting sperm from bacteria and other infections. But if their concentration gets too high - say, after an infection - it can also cause significant damage.

In the sperm of males whose partners have suffered miscarriage, the authors found four times more of these molecules. And while they still aren't sure why this is happening, this is one of the most important hints to date as to what could be behind RPL.

As research in this area slowly begins to mature, there is increasing evidence that obesity and old age can lower sperm health. As a result, the team is looking at whether either of these factors can somehow trigger a proliferation of these damaging molecules.

It's not a bad hunch given that, on average, the male participants in this study whose partners had experienced miscarriages were slightly older and more overweight than the controls. If it's true, however, it also means we have been consistently overlooking a potentially major factor behind miscarriage.

Fewer than half of all miscarriages in the US have an identifiable cause, but while women with RPL are routinely screened for risk factors, current guidelines do not recommend the same for male partners.

"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," says Jayasena.

"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."

This study has been published in Clinical Chemistry.