The North American Menopause Society (NAMS) estimates that 6,000 women daily and more than 2 million women each year enter menopause.² Menopause occurs in three stages: perimenopause, menopause, and post menopause. Perimenopause is when estrogen levels begin sporadically increasing and decreasing, leading to irregular menstruation and adverse symptoms, such as hot flashes. Perimenopause begins the transition to menopause and can last for 5 years or longer. Menopause is defined as the final cessation of menstrual periods and is confirmed after 12 consecutive months of amenorrhea. A woman is then considered to be postmenopausal. At this time, her ovaries produce very little estrogen and no progesterone, and ovulation has ceased. Typically, a woman experiences natural menopause between the ages of 45 and 54 years.

For clarification, this article uses the NAMS terminology: ET refers to unopposed estrogen therapy; EPT refers to combination estrogen and progestogen therapy; and the term progestogen refers to both progesterone and progestins. NAMS describes progesterone as being identical to the hormone produced by the ovaries, whereas progestins are synthesized compounds that act like progesterone. HT refers collectively to both ET and EPT.³

http://www.jaapa.com/a-new-verdict-for-hormone-therapy-ht-safe-for-younger-symptomatic-women/article/158743/