Women who have sex throughout their cycle may be better at fighting off illness while simultaneously increasing their odds of getting pregnant. (Photo: Getty Images)

It seems a no-brainer for women trying to conceive: You’re more likely to get pregnant when you have more sex.

But while experts have known that women who have sex throughout their cycle have a better chance of conceiving, they haven’t been able to pinpoint why — until now.

Two new studies from Indiana University published in the journals Fertility and Sterility and Physiology & Behavior studied menstrual cycle data from 30 women and determined that having sex throughout a woman’s cycle — even when she’s not ovulating — creates physiological changes that increase her odds of getting pregnant.

Researchers found that women who are sexually active have bigger changes in helper T cells (which manage the body’s immune response) as well as the proteins that helper T cells use to communicate.

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Among the findings: There were significantly higher levels of type 2 helper T cells (which help a woman’s body accept sperm and an embryo) in women who were sexually active during the luteal phase of their menstrual cycle, i.e. the period in which the uterine lining thickens in anticipation of pregnancy.

Researchers also discovered that sexually active women had higher levels of immunoglobulin G antibodies (which fight disease without interfering with the uterus) during the luteal phase.

As a result, they determined that the bodies of women who have sex throughout their cycle may be better at fighting off illness while simultaneously welcoming sperm or a fetus at the right moment.

Researchers didn’t detect these immunity changes in women who abstained from sex.

Lead study author Tierney Lorenz, PhD, a visiting research scientist at The Kinsey Institute, tells Yahoo Health that they’re not exactly sure why this immune response happens, but she has several theories.

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One is that sex might trigger changes in hormone patterns across a woman’s menstrual cycle or change her rates of ovulation. As a result, a woman’s autonomic nervous system may act differently over the course of her menstrual cycle.

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There may also be a link between exposure to bacteria from a woman’s partner and increased fertility. “Getting exposed to your intimate partner’s microbiome (the combination of bacteria, yeasts, and other very tiny organisms that live inside and on all of us) might challenge the immune system in ways that are different than if it were not exposed to that microbiome,” Lorenz says.

A man’s ejaculate may also stimulate or suppress the immune response in a woman’s reproductive tract or send a signal to the rest of her immune system to change in some important ways, she says.

But Lorenz also says there may be some other factor they’re just not aware of yet, such as a difference in diet, sleep, or social interactions among women who are and aren’t having sex.

So, what does all of this mean for women trying to conceive?

While Lorenz’s research found that the more frequently a woman has sex, the more often her immune system “gets the message that it’s time to reproduce,” she also points out that even a single act of sex that happens outside of a woman’s “fertile window” is still useful for increasing her fertility.

Her advice: “Do what works right in your relationship.”

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