These cases of ovulatory dysfunction are linked to infertility and to menstrual disturbances. During ovulation, cells are constantly stimulated to divide while ovulatory cycles continue. Amenorrhea (absence of menstruation) occurs in about 20% of women with ovulatory dysfunction. In the late follicular phase, women are the most fertile in the ovulatory cycle. Also, during the late follicular and ovulatory phases, women prefer the scent of masculine men. Infrequent and light menstruation occurs in about 40% of women with ovulatory dysfunction. However, it is unclear whether attraction to MHC heterozygosity changes across the ovulatory cycle. Indeed, studies have shown that women's preferences vary across the ovulatory cycle. This means they are considered monoautochronic ovulatory. Ovulatory periods are often accompanied by midcycle symptoms such as mittelschmerz or premenstrual symptoms.