The 3-5 year period before menopause when your estrogen and hormone levels begin to drop is called perimenopause. You typically enter into perimenopause in your late 40's and could begin to experience irregular menstrual cycles and symptoms such as:

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. A form of progestin therapy may also be an option to control menstrual bleeding and address vasomotor symptoms. Lifestyle changes are often recommended to help relieve other unpleasant symptoms.

Source: http://menopause.northwestern.edu/content/stages-menopause