Normal changes in the hormonal system occurring during puberty, pregnancy and menopause may increase the level of androgens or decrease the levels the female hormone estrogen, resulting in sudden new hair growth on the body or face. Menstrual irregularity, abdominal pain, signs of virilism, and abnormal appearance of secondary male sexual characteristics in a woman, in combination with new hair growth, signal a need for immediate medical evaluation. Medications such as steroids and birth control pills, or the surgical removal of sex glands may stimulate abnormal hair growth. Hair forms in a pouchlike structure below the skin called a hair follicle. What we see as hair is actually the hair shaft, which is the keratinized, hardened tissue that grows from this follicle.
People have more hair follicles per square inch than chimpanzees and gorillas.
Because most of the hair is fine and pale it usually isn't visible to the naked eye. Hair grows through three distinct growth phases. Active growth phase called the Anagen stage which can last several years. Regressive phase called the catagen stage which lasts about two weeks, during which the hair stops growing but is not yet shed. Resting phase called Telogen stage which lasts about 5-6 weeks, at the end of which the hair falls out and a new hair begins to form.