The announcement that sex leads to pregnancy is ten thousand-year-old news, but Indiana University scientists have given it a new spin by confirming the suspicion that sex throughout the menstrual cycle increases the chance of conception. Besides helping those struggling to conceive, the findings could have implications for wider understanding of the immune system.
"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," said Dr Tierney Lorenz in a statement.
It sounds like an excuse for people who really just want to have more sex – “look we need extra practice” – but Lorenz is the lead author of two papers released in the last month showing the advice has substance.
Sperm, and the fertilized egg, run the risk of being rejected by the mother's immune system as foreign objects, so it makes evolutionary sense that there might be a trigger to make the body's defenses less hyper-alert when pregnancy is a possibility.
Previous studies have produced contradictory results about changes to women's immune systems during the menstrual cycle. Lorenz suggests this may be because they failed to ask about sexual activity. Certain changes might be expected, “only in women who are reproductively active – that is, regularly engaging in sexual activity,” she and her co-authors write in Physiology and Behavior.
As part of the Kinsey Institute's Women, Immunity and Sexual Health study Lorenz tested biomarkers in two groups of women, half of whom were sexually active, throughout their menstrual cycle.
In Fertility and Sterility Lorenz reports that the mix of helper T cells ratios changed during the luteal phase, immediately after ovulation. Helper T cells mobilize the immune system cells responsible for killing material identified as foreign. Sexual activity increased levels of type 2 helper T cells (TH2) during this phase, while decreasing type 1 cells (TH1) relative to the rest of the cycle.
The Physiology and Behavior paper reports a similar change in the mix of immunoglobulin antibodies in saliva. Sexually active women had higher levels of the antibody IgG, common in blood. On the other hand levels of IgA, a different antibody the authors describe as the body's “first line of defense against invaders,” were lower among those having sex most often.
The combined effect is to temporarily give sexually active women an immune system more open to conception.
Lorenz told IFLScience “There were a number of very puzzling findings” from her research that will need more work to unravel. Most notably, she is not sure why IgG rises with sexual activity. She noted that the study was done by testing saliva, and said, “I do think we are seeing an example of immunoredistribution, when the immune system sends certain cells and antibodies to where they need to be, so when we look elsewhere we find them in smaller numbers.” Thus an increase in antibodies in saliva may reflect a decrease in the vaginal tract, making it easier for the fertilized egg to survive.
The sample sizes, 32 in one study and 30 in the other, are also small enough that the results should be treated with caution, despite achieving statistical significance.
"The female body needs to navigate a tricky dilemma," Lorenz said. "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."
"We're actually seeing the immune system responding to a social behavior: sexual activity," Lorenz added. "The sexually active women's immune systems were preparing in advance to the mere possibility of pregnancy."
Sexually active women experienced the same immune system changes when they used condoms, but to a lesser extent, leading Lorenz to suggest there might be “multiple mechanisms”, of which ejaculate exposure is only one.