fibroid while not largely affecting ovarian production of estrogen (and thus systemic levels of it). Aromatase overexpression is particularly pronounced in African-American women. Genetic and hereditary causes are being considered and several epidemiologic findings indicate considerable genetic influence especially for early onset cases. First degree relatives have a 2.5-fold risk, and nearly 6-fold risk when considering early onset cases. Monozygotic twins have double concordance rate for hysterectomy compared to dizygotic twins. Expansion of uterine fibroids occurs by a slow rate of cell proliferation combined with the production of copious amounts of extracellular matrix. A small population of the cells in a uterine fibroid have properties of stem cells or progenitor cells, and contribute significantly to ovarian steroid-dependent growth of fibroids. These stem-progenitor cells are deficient in estrogen receptor α and progesterone receptor and instead rely on substantially higher levels of these receptors in surrounding differentiated cells to mediate estrogen and progesterone actions via paracrine signaling. Diagnosis Physical examination and ultrasound are sufficient for diagnosing uterine fibroids in the majority of patients. When ultrasound findings are inconclusive, magnetic resonance imaging (MRI) may be able to confirm the diagnosis of uterine fibroids in most cases. In addition, MRI can identify benign uterine fibroids with atypical imaging features and fibroids with variant growth patterns. MRI can also identify other uterine (e.g. adenomyosis, endometrial polyps, endometrial cancer) and extrauterine (e.g. benign and malignant ovarian tumors, endometriosis) disorders that may mimic the appearance of uterine fibroids and/or contribute to the patient's symptoms. However, a small proportion of uterine fibroids can mimic other malignant uterine tumors (e.g. leiomyosarcoma) on all available imaging modalities (e.g. ultrasound, CT, MRI and PET-CT). Malignant tumors of the uterine wall (e.g. leiomyosarcoma) are very rare. Findings suggestive of a malignant uterine tumor rather than a benign fibroid include, fast or unexpected growth (particularly after menopause), interruption/effacement of the endometrial stripe, lymph node enlargement, invasion of adjacent organs and metastases to distant organs (e.g. lung). MRI findings suggestive of a malignancy include nodular/ill-circumscribed tumor margins, intermediate/high T2-weighted signal intensity of the solid tumor components, regions with high signal T1-weighted sequences in keeping with subacute hemorrhage, fine/wispy enhancement of the solid parts of the tumor, and restricted diffusion on diffusion-weighted imaging (DWI). A biopsy is rarely performed and if performed, is rarely diagnostic. Should there be an uncertain diagnosis after ultrasounds and MRI imaging, surgery is generally indicated. Coexisting disorders Fibroids that lead