Most female mammals experience “heat,” occasional periods when they can get —and as a result, become eager for . But human women don’t go into heat. They can become pregnant and feel receptive to sex year-round. Nonetheless, a good deal of research shows that in reproductive-age women, libido is to some extent cyclical across the menstrual cycle, with peak erotic occurring around the time of ovulation midway between menstrual periods. It’s evolution’s way of spurring procreation.

Libido Peaks at Ovulation: The Studies

University of Virginia, Charlottesville, researchers used standard surveys to chart libido and sexual functioning in 115 women, age 23 to 45. Each participant completed the survey twice, once mid-cycle around ovulation, and once pre-menstrually. The women reported significantly more interest in sex and greater satisfaction from at mid-cycle.

Dutch researchers showed erotic videos to 20 women whose genitals were wired to detect blood flow. Increased blood flow is an indicator of physiological arousal. Genital blood flow increased the most around the time of ovulation, and in an accompanying survey, the women said they felt most aroused at that time of the month.

Researchers at the University of Rhode Island recruited 96 college-age women who were asked to rate their interest in four types of films: comedies, romances, action-adventure, and erotica. The researchers recorded a "surge” in interest in the erotic films as the women approached ovulation.

Arizona State University researchers asked 236 women to keep diaries tracking their . They also recorded their basal body temperature, which spikes at ovulation. Masturbation peaked around ovulation.

Researchers in Canada videotaped 19 women as they walked down a street during ovulation and menstruation. Thirty-five men viewed the videos and rated the women’s sexual . Compared with the menstrual women, the ovulating group swayed their hips more, and the men rated them sexier.

Swedish researchers have reported that women’s and interest in sex, erotic art, and buff, muscular men all increase around the time of ovulation.

Finally, Spanish scientists reviewed the literature on sex and the menstrual cycle, concluding that “women at mid-cycle [ovulation] exhibit increased sexual motivation.”

But studies of libido across the menstrual cycle are not unanimous. Australian researchers asked 173 college-age women to rate their arousal to various sexual fantasies. The women showed no significant cyclical differences. In fact, the researchers noted “a high level of stability [in arousal] across the menstrual cycle.”

Nonetheless, the weight of the evidence suggests that women experience a slight-to-moderate libido increase around ovulation.

Why Mid-Cycle "Heat"?

It makes sense for women to be most sexually receptive around ovulation. The biological purpose of life is to reproduce life, so evolution has apparently primed women to feel most sexually aroused when they’re most likely to get pregnant.

In addition, pre-menstrual and menstrual days can cause discomfort that turns women off to sex. Studies vary but somewhere around half of women experience pre-menstrual upsets—irritability, , and blues—and every month, about half of reproductive-age women experience menstrual cramping. PMS and cramps tend to suppress interest in sex.

Confounding Factors

The menstrual cycle may nudge women to feel more sexual around ovulation, but many other factors also influence women’s libido:

The Pill. contraception alters the natural menstrual cycle. A few of the studies showing cyclic libido divided participants by their use of birth control pills. Women on the Pill showed no monthly libido changes, but those not taking hormonal contraception did.

Work. Studies of women at work show little cyclic change in libido and decreased interest in sex in general. But while on vacation, these same women experienced libido rebound and cyclic erotic interest became more evident.

A long-term relationship. Compared with women in established couples, single women show a greater ovulatory spike in sexual interest. This lends some credence to an old joke among men: What single thing can a man do to destroy his girlfriend’s interest in sex? Marry her.

The Upshot

While women’s interest in sex may not vary much throughout the month, if it does, women might track their own erotic thoughts and feelings, and plan romantic evenings or getaways accordingly.

Meanwhile, not many men follow the menstrual cycles of the women they’re involved with. Here’s a reason to. You’re most likely to get lucky during her mid-cycle. But always remember, the menstrual cycle is not destiny.

While the research shows a surge in women's sexual interest around ovulation, it's subtle, so I'd love to hear from women about this. Do you notice cyclic erotic ups and downs? What’s been your experience?