More frequent sex increases the chances of a successful pregnancy. Makes sense. But not for the reasons you think! Two newly published research articles discuss studies of heterosexual intercourse outside the fertile window and come to similar hypotheses.
The results showed that couples having intercourse outside the fertile window increased their chances of pregnancy compared to heterosexual couples who abstained from sex when it was not the woman’s most fertile time of the month. The researchers’ theory is that intercourse causes a shift toward a more pregnancy-friendly immune response.
In the simplest terms, your immune system has two types of immunity: cellular immunity (Th1), the part of your immune system that attacks viruses and tumors, and humoral immunity (Th2), which creates antibodies and attacks bacteria and invaders outside a cell. A down-regulation of Th1 and up-regulation of Th2 is necessary for a successful pregnancy. When a woman is pregnant, her body shifts toward a Th2 response (which is why women are told to get a flu vaccine, since it’s the Th1 system that prevents the flu).
The studies’ findings suggest that sex causes a shift toward a Th2 response, which may prime a woman’s immune responses so that when she does have sex during her fertile window, her immune system is already engaged in pro-pregnancy responses.
There are limitation to these studies, though. They look only at heterosexual intercourse and do not take into account other feel-good hormones such as oxytocin that are expressed during sex. But the takeaway would be for partners to enjoy each other in whatever way works for their relationship and not be concerned about sex after ovulation, IUI, or IVF.