The 3-5 year period before menopause when your estrogen and hormone levels begin to drop is called perimenopause. During this transition phase, leading up to menopause, the supply of mature eggs in your ovaries diminishes and ovulation becomes irregular. At the same time, the production of estrogen and progesterone decreases.
It is the big drop in estrogen levels that causes most of the symptoms like hot flashes, vaginal dryness, sleeplessness and changes in libido. Hot flushes and night sweats can start five or more years before your periods stop at menopause. There is still a chance that you could get pregnant during this time and if you want to avoid this, a form of birth control is recommended until one year after your last period.