Previous research suggests having sex boosts immunity and that doing so during the optimal ovulation window improves a couple’s likelihood of conceiving. But a new study sheds light on the relationship between those findings— and alters the second, suggesting an increased chance of fertility regardless of when a woman has sex.

Researchers at Indiana University found that sexual intercourse, even when performed outside the window of ovulation, causes physiological changes in a woman’s body that make her more fertile.  

"It's a common recommendation that partners trying to have a baby should engage in regular intercourse to increase the woman's chances of getting pregnant— even during so-called 'non-fertile' periods— although it's unclear how this works," study author Tierney Lorenz, a visiting research scientist at the Kinsey Institute at Indiana University, said in a news release. "This research is the first to show that the sexual activity may cause the body to promote types of immunity that support conception.

"It's a new answer to an old riddle: How does sex that doesn't happen during the fertile window still improve fertility?"

While earlier studies suggest that immune function changes during pregnancy, as well as after childbirth and throughout the menstrual cycle, authors of the current study say their findings are the first to illustrate the effect of sex itself on these changes as well as on immune system regulation.

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Study authors’ conclusions, which were published recently in the journal Fertility and Sterility and the journal Physiology and Behavior, stem from data based on 30 healthy women’s menstrual cycles. About half of those women were sexually active, while the other half were sexually abstinent.

Findings in the first paper suggest that, compared to abstinent women, sexually active women see greater changes in helper T cells and the proteins that T cells use to communicate. In the second paper, researchers found that levels of antibodies, or immunoglobulins, also varied between the two groups of women. Both of the shifts observed in the sexually active women, but not in the abstinent women, helped increase the chance of fertility.

While T cells help manage the body’s immune response, immunoglobulins help fight foreign invaders in the body to help prevent infection.

"The female body needs to navigate a tricky dilemma," Lorenz said in the release. "In order to protect itself, the body needs to defend against foreign invaders. But if it applies that logic to sperm or a fetus, then pregnancy can't occur. The shifts in immunity that women experience may be a response to this problem."

In the sexually active women, researchers observed higher levels of type 2 helper T cells during the luteal phase of the menstrual cycle, a period when the uterine lining thickens to prepare for pregnancy. This type of T cell prevents the body from detecting the fetus and sperm as a foreign body, thus allowing for impregnation.

Among the sexually active women, study authors also saw higher levels of type 1 helper T cells during the follicular phase in the menstrual cycle, when the ovaries’ follicles are maturing. Type 1 helper T cells help the body ward off outside threats.

Researchers similarly saw changes in immunoglobulin levels that aided in conception among the sexually active women. Levels of immunoglobulin A antibodies, which usually reside in the mucous of the female reproductive tract and can affect fertilization, were higher during the follicular phase among this group of women. Also, levels of immunoglobulin G antibodies, which are usually found in the blood and help ward off disease without affecting the uterus, were greater in the sexually active women during the luteal phase.

Study authors noted that none of those shifts, all of which prime the female body for a greater chance of fertilization, occurred in the women who did not have sex.

"We're actually seeing the immune system responding to a social behavior: sexual activity," Lorenz said in the release. "The sexually active women's immune systems were preparing in advance to the mere possibility of pregnancy."

Lorenz and her colleagues said that, among other implications, their findings could impact doctors’ recommendations to couples looking to conceive.