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Scientists say they’ve figured out why having frequent sex can increase a woman’s chances of getting pregnant, even when she is outside the window of ovulation.

The findings could eventually change recommendations regarding how often to engage in sexual intercourse for couples trying to get pregnant and could also have an impact on how to treat people with autoimmune disorders.

“It’s a common recommendation that partners trying to have a baby should engage in regular intercourse to increase the woman’s changes of getting pregnant—even during so-called ‘non-fertile’ periods—although it’s unclear how this works,” says Tierney Lorenz, a visiting research scientist at the Kinsey Institute at Indiana University. “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?”

Earlier studies have shown changes in immune function during pregnancy and after childbirth and changes in immunity across the menstrual cycle. But the new research, published in the journals Fertility and Sterility and Physiology and Behaviors, is the first to show that sexual activity plays a role in these changes with clear differences found in immune system regulation in women who are sexually active versus women who are sexually abstinent.

The results are based upon information from participants in the Kinsey Institute’s WISH Study—Women, Immunity and Sexual Health—which collected data across the menstrual cycle in 30 healthy women, about half of whom were sexually active and half of whom were sexually abstinent.

In the first paper, report sexually active women experienced greater changes in helper T cells, and the proteins that T cells use to communicate. In the second paper, they report differences in antibody levels between the two groups.

Helper T cells manage the body’s immune response in part by activating the cells that destroy invading microbes in the body. The antibodies—also known as immunoglobulins—are secreted by white blood cells and play an important role fighting off foreign invaders in the body.

“The female body needs to navigate a tricky dilemma,” Lorenz says. “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.”

Immune system isn’t passive

There are several types of helper T cells and immunoglobulins. Type 1 helper T cells assist the body with defense against outside threats. Type 2 helper T cells help the body accept those aspects of pregnancy the body may otherwise interpret as “foreign invaders,” such the presence of sperm or emerging embryo.

Similarly, immunoglobulin A antibodies—typically found in the mucous of the female reproductive tract—can interfere with the movement of sperm and other aspects of fertilization. Immunoglobulin G antibodies—typically found in the blood—fight disease without interfering with the uterus.

The researchers found significantly higher levels of type 2 helper T cells in sexually active, non-pregnant women during the luteal phase of the menstrual cycle, a period when the uterine lining thickens in preparation for pregnancy. Higher levels of type 1 helper t cells were found these same women during the follicular phase in the menstrual cycle, a period when the ovaries’ follicles are maturing.

They also found sexually active women experienced similar changes in immunoglobulins, with higher levels of immunoglobulin G during the luteal phase and higher levels of immunoglobulin A during the follicular phase.

Neither shifts in immunity were observed in the sexually abstinent women.

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

Both studies contribute to a growing body of evidence that the immune system isn’t a passive system that waits to react to outside threats, but a highly proactive system that changes in response to external cues, such as the physical environment and social behavior.

The studies may also shed light on previous research that found unexplained fluctuations in immune response in women. A recognition that sexual activity can cause natural fluctuations in blood tests results could be useful to physicians treating patients with immune disorders, Lorenz says.

The National Institutes of Health’s National Institute of Child Health and Human Development, American Psychological Foundation’s Henry P. David Award for Research in Human Reproductive Behavior and Population Studies, and Office of the Vice Provost for Research at IU Bloomington funded the work.

Source: Indiana University