Medical doctors and researchers do not fully understand what causes uterine fibroids, but it is generally believed that the following contributes to their development: • Extracellular matrix (ECM), which is a material in your body that helps cells stick together and is increased in fibroid tissue • Development of certain growth factors • Genetic mutations, as some fibroids contain genetic tissue that is different from normal uterine cells • Hormonal imbalances of progesterone and estrogen