For women who are trying to get pregnant, timing sex so that it falls during the optimal ovulation window is typically the focus. But new research suggests that having sex - at any point during a woman's cycle - sparks changes in the immune system that increase the likelihood of conception.
This is the conclusion of two studies published in the journals Fertility and Sterility and Physiology and Behavior - both led by Tierney Lorenz, of the Kinsey Institute at Indiana University.
Lorenz notes that couples trying to conceive are typically advised to have regular sex to increase the chances of getting pregnant, even during "non-fertile periods," but it is not understood how this works.
"It's a new answer to an old riddle: How does sex that doesn't happen during the fertile window still improve fertility?" she says.
To further explore this topic, Lorenz and her colleagues used information from participants in the Kinsey Institute's Women, Immunity and Sexual Health (WISH) Study, which accumulated data across the menstrual cycles of 30 healthy women - half of whom were sexually active and half of whom were not.
'Female body navigates a tricky dilemma'
In one paper, the team describes how the sexually active women displayed more changes in helper T cells, which manage the body's immune response by activating cells that demolish microbes invading the body.
In the second paper, they present evidence of differences in antibody levels between the two groups of women. The antibodies, which are known as immunoglobulins, play a critical role in destroying foreign invaders in the body.
Explaining further, Lorenz says:
"The female body needs to navigate a tricky dilemma. 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."
The researchers point out that there are several types of helper T cells and immunoglobulins. While type 1 helper T cells help the body defend itself from outside threats, type 2 helper T cells assist the body in accepting conditions of pregnancy that the body would normally interpret as a threat - such as sperm or an embryo.
Additionally, immunoglobulin A antibodies can hinder sperm movement and other features of fertilization. Meanwhile, immunoglobulin G antibodies can fight disease without affecting the uterus.
Results from their research show that sexually active, non-pregnant women had significantly higher levels of type 2 helper T cells during the luteal phase of their menstrual cycle - when the uterine lining thickens in anticipation of pregnancy.
The team also found that these same sexually active women had higher levels of type 1 helper T cells during the follicular phase of their cycles, when the ovaries' follicles are maturing.
What is more, the sexually active women displayed similar changes in immunoglobulins; they had higher levels of immunoglobulin G during the luteal phase and higher levels of immunoglobulin A during the follicular phase.
In the group of sexually abstinent women, none of these immunity changes were present.
'Immune system responds to a social behavior'
The researchers say their studies are the first to show that sexual activity initiates changes in immune system regulation in sexually active women, compared with sexually abstinent women.
"We're actually seeing the immune system responding to a social behavior: sexual activity," explains Lorenz. "The sexually active women's immune systems were preparing in advance to the mere possibility of pregnancy."
These findings could potentially influence recommendations for couples trying to get pregnant, in terms of how often they should have sex.
Likewise, the team explains their research could impact treatment for people with autoimmune disorders. If sexual activity causes natural fluctuations in blood tests, it could be useful for physicians treating such patients.
Medical News Today recently reported on a study in which scientists were able to map the gene activity of a human embryo's first days after fertilization, potentially opening up new avenues for fertility treatments.