Epidemiologic studies over the past 40 years have identified numerous risk factors for breast cancer, including: older age, an early age at menarche, a late age at first full-term birth, not having children, a family history of breast cancer in a first-degree relative, greater height, higher circulating estrogen levels, postmenopausal hormone use, breast density, history of breast biopsies, obesity (for postmenopausal breast cancer), and exposure to ionizing radiation.