Oral sex has been linked with reduced incidence of recurrent miscarriage, according to a new study conducted in the Netherlands.

Recurrent miscarriage, in which a woman experiences three or more pregnancy losses prior to the 20th week of gestation, affects about 1 percent of women, although some estimates are higher.

The causes of recurrent miscarriage can vary, including uterine anomalies, endocrine disorders, maternal inherited and acquired thrombophilia, and parental chromosomal abnormalities.

But for many couples, the reason behind multiple episodes of pregnancy loss is never actually identified.

According to a Dutch team of obstetricians from Leiden University Medical Centre, one of our blindspots in relation to recurrent miscarriage could be because most existing research into the immunology of the phenomenon is based on the maternal immune system.

Paternal factors, they say, haven't been explored as much, even though existing research suggests men – and especially their semen – can affect the female immune system prior to conception, and after it also.

The thinking goes that foetus-specific maternal tolerance to paternal antigens can be induced via exposure to sperm – and not just vaginal exposure, but oral exposure.

One study in 2000 found that oral sex and the act of swallowing sperm during it was correlated with a diminished occurrence of preeclampsia in women.

Of course the research isn't in anyway saying people should or shouldn't be giving oral sex – but the Leiden team were interested to dig into the biology behind the link further.

In this study, they looked at 97 women who had experienced at least three unexplained consecutive miscarriages and who were under 36 at the time of the third miscarriage.

When matched with a control group – consisting of 137 women who hadn't lost recurrent pregnancies – questionnaire results indicated that the women who hadn't lost any pregnancies were performing significantly more oral sex on their male partners on average.

In the analysis, a bit over half (56.9 percent) of the miscarriage group reported having oral sex with their partners – significantly lower than the almost three-quarters (72.9 percent) of the non-miscarriage group who reported oral sex.

These results don't show any kind of causation link – there is no evidence at all that oral sex is what's contributing to this reduction in miscarriages. There are multiple other factors at play and this study only looked at one tiny part of a relationship, and only via self reporting. 

But the researchers say it provides evidence that the way females are exposed to semen and the sexual activities they're partaking in with their partners could be a factor to research further as we try to understand why miscarriages happen – and hopefully find a way to reduce them in future.

"This matched case control study suggests that women with recurrent miscarriage had less oral sex compared to women with uneventful pregnancy," the authors explain.

"This is in line with the hypothesis that the gut has the most adequate absorption in the absence of an inflammatory environment, and seminal fluid contains soluble HLA antigens which can already induce maternal immune tolerance towards inherited paternal antigens of the foetus before implantation."

The researchers say vaginal exposure to paternal antigens is also a factor involved in successful pregnancies, and suggest that the findings could be explained by immunoregulatory factors contained in seminal fluid, such as cytokines, hormones, and soluble HLA (sHLA) antigens, which could play an important function in creating maternal tolerance that influences foetus survival during pregnancy.

The researchers acknowledge their relatively small study needs a larger follow up, and admit their findings remain only hypothetical for now.

Nonetheless, they suggest oral exposure to seminal fluid could induce maternal tolerance to paternal antigens, and may therefore "influence pregnancy outcome in a positive way".

The findings are reported in the Journal of Reproductive Immunology.