Heterosexual couples trying to start a family have tools to tell them when it's time for baby-making sex: apps can track a woman's cycle; over-the-counter tests can pinpoint ovulation. But it turns out the sex they're having the rest of the month could be just as important for starting that bundle of joy.
That's because as far as a woman's immune system is concerned, both sperm and new embryos are invaders to be destroyed. According to two new papers from the Kinsey Institute's WISH Study, regular sex seems to push the female immune system into a gentler mode that protects sperm and tolerates the embryo.
In these studies, lead author Tierney Lorenz examined how the numbers of available antibodies and immune system cells changed inside a woman over one monthly cycle. She compared her results from heterosexual women with a regular sexual partner with measurements from women who remained abstinent.
She found that when women who were having vaginal intercourse ovulated, they also lowered the number of IgA antibodies in their bodies. Normally, IgA antibodies guard epithelial linings like the inner surface of the vagina and uterus, grabbing onto foreign invaders like bacteria, viruses, and sperm. When IgA levels drop, they leave another type of antibody, called IgG, in charge: IgG antibodies work inside blood and other body fluids, providing a line of defence that does not target the sperm inside the reproductive tract. The more sex the women had each week, the lower their IgA levels got.
And when these women reached the part of their cycles when a new embryo might implant in the uterus, they also cut back on the number of immune cells that attack viruses and bacteria. The change left behind higher numbers of immune cells that can tolerate the embryo and improve its chances of implanting in the uterine lining.
These changes did not occur in women who were abstinent through the study, suggesting that having sex somehow sends a signal to the female body that changes how the immune system behaves. At this point, no one knows exactly what that signal is. According to Lorenz,
There are a lot of ways that sexual activity might influence the immune system. Sex might trigger changes in hormone patterns across the menstrual cycle, or change rates of ovulation. It might change the way that the autonomic nervous system acts over the course of the menstrual cycle. Getting exposed to your intimate partner's microbiome might challenge the immune system in ways that are different than if it were not exposed to that microbiome. There might be something in ejaculate that stimulates or suppresses immunity in the female reproductive tract, or sends a signal to the rest of the immune system to change in some important way. Or there may be some other factor that is different between women who are and are not having sex that is relevant to any and function, such as diet, sleep, or simply social interactions. There is a lot of research that needs to be done.
These studies were small, and only looked at immune changes over a single cycle in women who were in their early 20s, so there are a lot of variables they don't take into account. It's not yet known whether older women or women who have already had children will have similar responses, or even whether the effect will persist at different times of the year.