The transition from ovulatory cycles to menopause typically begins in the late 40s and in early menopausal transition. Levels of FSH rise slightly and lead to an increased ovarian follicular response. An FSH level greater than 40 mIU/mL has been used to document ovarian failure associated with the menopause. Estrogen levels may be normal, elevated, or low depending on the stage of menopausal transition. Only at menopause are estrogen levels extremely low or undetectable. Ref: Hoffman B.L., Schorge J.O., Schaffer J.I., Halvorson L.M., Bradshaw K.D., Cunningham F.G., Calver L.E. (2012). Chapter 21. Menopausal Transition. In B.L. Hoffman, J.O. Schorge, J.I. Schaffer, L.M. Halvorson, K.D. Bradshaw, F.G. Cunningham, L.E. Calver (Eds), Williams Gynecology, 2e.