A newborn female infant has over a million oocytes; the oocyte cohort shrinks throughout life such that there are only a few thousand oocytes left as a woman enters her forties and few or none in the postmenopause. It is the depletion of oocytes which eventually leads to the cessation of menstruation, the cardinal sign of the menopause. There are two landmarks in the ovarian failure process. First, there is a marked decline in fertility with no cycle dysfunction. Subsequently, cycle changes become noticeable as the follicular phase shortens and luteal phase dysfunction occurs.
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