The Menopausal Transition:

In 2001, The Stages of Reproductive Aging Workshop (STRAW) staging system was developed to describe various stages of the menopausal transition (Table 1). On average, the menopausal transition lasts 4 years in duration and is divided into early and late phases

(1). It begins when menstrual irregularity first appears, classically defined as either a "skipped" period or by an increase in variability of cycle length by more than 7 days. It is well established that the greatest number of follicles are present in utero in the fetus at 20 weeks of gestation. These follicles undergo atresia, with a more rapid rate of decline after the 3rd decade of life

(2). The menstrual irregularity that characterize the menopausal transition occurs as this overall follicular complement decreases, which in turn leads to a decrease in the number of ovarian follicles available to grow and potentially ovulate in any given cycle. The result is a decrease in inhibin B production.

This reduction in inhibin B leads to an increase in follicle-stimulating hormone (FSH) secretion. Early in the transition period, FSH levels are not consistently elevated, and may often vary considerably from month to month as the growing follicle cohort itself varies month to month. 

The follicular phase becomes notably shorter, and as a result, estradiol (E2) production is variable and even elevated at times. Anovulatory cycles become more prevelant, and luteal progesterone production declines. 

This early phase of the transition is associated with an increase in menopausal symptoms such as hot flashes, though initially, the increase may be relatively small as there may not necessarily be a reduction in the amount of circulating E2. 

By the late transition, prolonged amenorrhea (defined as > 60 days) occurs, and is associated with a persistently reduced follicle pool and failure of folliculogenesis. At this point in the transition, estrogen deficiency begins to dominate, bone mineral density loss begins and menopausal symptoms including hot flashes and vaginal dryness will increase dramatically.