Premature menopause occurs when ovaries stop working at a young age, less than 40. This is at least 10 years earlier than the average age of the menopause. Because of the loss of ovarian follicular activity, levels of the female hormone, oestrogen, drop to very low levels.
Ovarian activity reflects the activity of the follicles that make up the ovaries. Each follicle consists of an egg surrounded by an outer covering of cells known as follicular cells. The egg is important for fertility whereas follicular cells produce hormones. Monthly development of ovarian follicles leads to the release of an egg (ovulation) and to the production of the two female hormones, oestrogen and progesterone. These hormones act on the womb lining causing it to thicken and then to break down, and are therefore responsible for bringing about menstrual bleeding. Females are born with a fixed quota of ovarian follicles (~1-2 million), the numbers of which become less and less throughout life. Eventually, follicle numbers drop to very low levels (less than 1000) leading to a fall in circulating hormone levels, the stoppage of periods, and consequently, the menopause.
Premature menopause therefore represents the transition into an oestrogen-deficient and infertile state at an early age.
For more information on follicles, hormones and ovulation see my sections on The Menstrual Cycle and Ovulation Cycle Tracking and Anovulation and Ovulation Induction.