A new report shows that sexually active women have a much better chance of getting pregnant than those who remain abstinent, a press release from Eurekalert reports.

Researchers from Indiana University have shown that sexual activity leads to physiological changes in the body of the woman that increases her chances of getting pregnant, even outside the window of ovulation.

The report was recently published in the journal Fertility and Sterility and the journal Physiology and Behavior.

The lead author on both papers is Tierney Lorenz, a visiting research scientist at the Kinsey Institute. Julia R. Heiman, a professor in the Department of Psychological and Brain Sciences, and Gregory E. Demas, a professor in the Department of Biology, are also co-authors on the study.

"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," Lorenz said. "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. However, IU research is the first to show that sexual activity plays a role in these changes in immune system.

In the first paper, Lorenz and colleagues showed that sexually active women experienced greater changes in helper T cells and the proteins that T cells use to communicate, while in the second paper, they showed differences in antibody levels between sexually active and 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."

The conclusions could influence advice on how often to engage in sexual intercourse for couples trying to get pregnant. It could also help treat people with autoimmune disorders.