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updated 7/1/2015 11:45:48 AM ET 2015-07-01T15:45:48

Women's cheeks get redder when they are the most fertile, but this color change is so subtle that it is undetectable by the human eye, a new study finds.

In the study, researchers in the United Kingdom took photographs of 22 women every weekday over the course of one month. After analyzing the photos, the researchers concluded that the women's faces got redder around the time they ovulated.

"This is the first study to conclusively show that women's faces do change in redness over the course of the menstrual cycle," said Robert Burriss, a co-author of the study and a research fellow in psychology at Northumbria University in England.

But the researchers had expected to find a change in skin redness that was detectable to the human eye, because this might explain why previous studies had suggested that men perceive women's faces as more attractive when the women are ovulating, Burriss told Live Science. Surprisingly, however, the study found that the changes in redness are too small for people to perceive, he said.

Past research had shown that men find women more attractive when the women are ovulating, and that men rate changes in women's voices and body odor during ovulation as more attractive. This new study investigated whether changes in women's facial coloring during their menstrual cycle might also explain how attractive they appear to men during ovulation.

The researchers used a specialized camera to take the photographs, and analyzed changes in the women's skin color and skin luminance, which is a measure of its lightness or darkness, at two locations on the women's cheeks. [ 5 Myths About Women's Bodies ]

The study found no change in the women's skin luminance, but there were variations in facial color, according to the findings published today (June 30) in the journal PLOS ONE. The redness in a woman's cheeks increased in the days before she ovulated and remained high until she began her next period, and then decreased rapidly after she started her period.

Attractiveness and fertility

Research has shown that men consider red facial skin more attractive, and researchers think the reason for this may be that the color suggests good health and youthfulness, Burriss said. The color of skin on other primate species changes in a similar way when they are the most fertile, he noted.

For example, female chimpanzees have swollen red bottoms at peak fertility as a clue to attract male chimps looking to mate.

But humans might not "advertise their fertility" the way that chimps and other species do, Burriss said. The benefit of concealing ovulation in women might be that it promotes relationship commitment, he noted.

"If men don't know when women are fertile, they stay interested in the relationship," Burriss said. In species where the timing of ovulation is obvious, such as chimps, males often ignore nonfertile females, he noted.

Some studies have suggested that women may advertise their fertility by becoming more flirtatious, but they do so only with men they find attractive. Other research has shown that women may make more attempts to enhance their appeal near the time of ovulation by wearing more revealing, stylish or red clothing.

A next step in this current research may be to determine whether skin-color changes are more pronounced in some parts of the face than in others during ovulation. For instance, future studies could look at whether the lips have more dramatic color changes than the cheeks do, Burriss said.

One weakness of the study is that it included mostly Caucasian women, so it's unclear whether similar patterns of skin variation during the menstrual cycle would be observed in women of different skin colors.

© 2012 LiveScience.com. All rights reserved.

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