Study: Frequent sex may raise chances for conception

INDIANAPOLIS — From a purely scientific point of view, a woman's reproductive and immune systems would appear to be at odds with each other.

The immune system shouts "out, out, out" to potential intruders. In order for a woman to conceive, though, her reproductive system must cordially invite in some very critical intruders: sperm.

Scientists knew that a woman's immune system changes during different parts of her menstrual cycle to make it possible for her to conceive. But they didn't know what triggers those changes.

As an Indiana University researcher strove to answer this evolutionary puzzle, she also wound up discovering what could be useful information for couples who are having trouble conceiving.

Conventional wisdom has held that couples who want to conceive should concentrate on having sex during the days of the month that the woman is fertile, "saving" themselves for that crucial potential conception time.

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But Indiana University scientist Tierney Lorenz found that such a tactic may actually hamper a woman's ability to conceive — because of her immune system. Having frequent sex, however, primes a woman's immune system to undergo changes that open the door for conception, Lorenz found.

“Women who are trying to conceive are told essentially that sex outside of that window doesn’t count,” said Lorenz, a visiting research scientist at the Kinsey Institute. "It’s certainly true that you absolutely have to have sex on the day that egg is present, but we shouldn’t discount sex on other days. ... Even if she doesn’t get pregnant in that cycle, it sets her up for the next cycle.”

Working with colleagues at the Kinsey Institute and Indiana University’s Center for the Integrative Study of Animal Behavior, Lorenz collected blood and saliva samples from 30 women at about four points in their menstrual cycle. About half were sexually active and half abstinent.

Women who were sexually active had greater variations in their helper T cells, which are specialized to attack certain types of invaders, across their cycle, Lorenz found. Some T cells allow for invaders such as sperm, while other types work hard to prevent them.

"If you lock into being one kind of helper T cell, you can’t be one of the other kind,” she said, “so the immune system has to make a decision, what do I prioritize now?”

Over the course of a woman’s cycle, women who were sexually active during their fertile period had many more of the type of helper Ts that would not be programmed to attack sperm or an embryo. During non-fertile periods, they had many T cells that would prevent such invaders.

Sexually active women also had shifts in the level of their antibodies, or immunoglobulins. Lorenz looked at immunoglobulin A antibodies, which reside in the mucous of the female reproductive tract and can block sperm from moving. She also studied immunoglobulin G antibodies, which are in the blood and have little impact on the reproductive tract.

During a woman’s fertile period, immunoglobulin G antibodies, which would allow for conception, are in higher supply. Once she ceases to be fertile, the balance of antibodies shift.

But neither of these variations applies to sexually abstinent women, Lorenz found.

“The benefit of having these shifts across the cycle, it allows the body to change its priorities at different points,” Lorenz said.

The researchers report their results in recent papers in the journals Fertility and Sterility and Physiology and Behavior.