Recreational sex — sex with no procreational purpose — might have a positive role in conception. Research from the University of Adelaide in South Australia shows that plenty of sex with the same man — even a year before conception — can increase the chance of a healthy pregnancy, reducing the risk of miscarriage, still births and pre-eclampsia.

An article in New Scientist discusses that these fertility problems might occur due to “the reluctance of the mother's immune system to accept the fetus and placenta”, which both express foreign proteins from the father's genes. “Sex, early and often, and with the intended father, may help overcome that reluctance”.

As a report from ABC NewsOnline explains, “The more accustomed the woman's immune system is to the man's sperm, the less likely her body will be to reject the fetus”. Repeated exposure to the man's semen, and the foreign proteins it contains, might help the mother-to-be to develop tolerance to them. Gustaaf Deekker from the University of Adelaide said, “If there's repeated exposure to that signal, then eventually when the woman conceives, her [immune] cells will say 'we know that guy, he's been around a long time, we'll allow the pregnancy to continue'”.

The Adelaide group has also identified the cytokine transforming growth factor (TGF)-β as a component of semen, which they believe is also important for tolerance, and might represent a new treatment for women who repeatedly suffer pregnancy failures.