The modern generation of women, with successful careers and high stress levels to match, could find their super-powered lifestyle is affecting their fertility.

An international comparison of women in 37 different populations and cultures shows that career women are more likely to look like Keira Knightley than Marilyn Monroe, even though the hourglass figure has previously been linked with improved fertility.

The research, published in the recent issue of the journal Current Anthropology, examines the shape of women around the world by comparing their waist-to-hip ratio, which is calculated by dividing a woman's waist circumference by the circumference of her hips.

Professor Elizabeth Cashdan, an anthropologist at the University of Utah, said there is evidence the hormonal profile linked to a slim-waisted, non-curvy shape favors women in “resource competition, particularly under stressful and difficult circumstances.”

The study suggests women who experience heavy work stress and are driven to succeed suffer a hormonal shift with their estrogen levels affected by increases of androgens — hormones linked to competitiveness and strength.

Cashdan said a small waist and large hips have significant fitness benefits for women.

Those women with a waist-to-hip ratio of 0.7, meaning their waist is 70 percent the size of their hips, are less prone to chronic disease and are typically considered to be more attractive.

For a comparison, the study looked at the measurements of 240 Playboy centerfolds and found they have an average waist-to-hip ratio of 0.68.

The average female waist-to-hip ratio is 0.82.

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