Ovulating girls have more sexual fantasies
The research is one of many studies finding differences in women's sexual interest across the menstrual cycle.
For example, a 2007 study found that around ovulation, when pregnancy is possible, women say they prefer macho, masculine guys.
An April 2011 study even suggested that women who are in the more fertile phase of the month are more likely to see Georgia O'Keeffe's suggestive paintings as erotic.
The new study finds that sexual fantasies increase, and lead to more arousal in women, during fertile periods. Women also reported a higher proportion of men in their fantasies during fertile times of the month.
"When it mattered most, women were fantasizing more about men," said study author Samantha Dawson, a graduate researcher at the University of Lethbridge in Alberta, Canada.
Dawson and her colleagues focused on fantasies because such sexual daydreams aren't dependent on the availability of sex partners or other outside forces.