Menstrual Cycle Duration
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The menstrual cycle occurs in humans and non-human, anthropoid primates. Like the oestrous cycle of other mammals, events of the menstrual cycle reflect ovarian cyclicity but the starting days of the ovarian and menstrual cycles differ. The first day of vaginal discharge of blood is considered Day 1 of the menstrual cycle. The entire menstrual cycle includes three phases. The first phase, menstruation, is the interval during which micro- or macroscopically detectable blood is discharged vaginally due to the break-down and shedding of the endometrium. Endometrial breakdown is triggered when ovarian steroid hormone levels fall with the demise of the corpus luteum in the absence of pregnancy. Menstruation is followed by a proliferative phase of the cycle during which the endometrium is regenerated which, in turn, is followed by the final, secretory phase of the cycle.
Menstrual cycle length is given as the time from the first day of bleeding to the first day of the subsequent event of bleeding. The human menstrual cycle is 28 days long on average, but it can range from 21 – 35 days. Great ape menstrual cycles appear to be more regular and generally longer compared with humans. Orangutans cycles are approximately 29 days, gorillas 30 – 32 days, bonobos 32-35 days and chimpanzees ~37 days, though it can range from 31 – 36.7 days in this species. The difference in cycle length correlates with the amount of time each great ape species is sexually fertile. Lack of cycling in orangutans over long periods is correlated with limited food supply. Cycling in all species is subject to influences from social stress and nutrition. Among the Old World Monkey species, the patas monkey cycles range from 24 – 27 days, vervet monkeys from 30 – 31 days, mangabys from 30 – 34 days and baboons from 30 – 35 days (Dixon, 1998 and references therein).