Several small studies have found that fertile women appear more attractive to men than women during infertile portions of her menstrual cycle, or women using hormonal contraception. It has also been suggested that a woman's voice may become more attractive to men during this time. Two small studies of monogamous human couples found that women initiated sex significantly more frequently when fertile, but male-initiated sex occurred at a constant rate, without regard to the woman's phase of menstrual cycle. It may be that a woman's awareness of men's courtship signals increases during her highly fertile phase due to an enhanced olfactory awareness of chemicals specifically found in men's body odor.