What causes fibroids? It’s unclear why fibroids develop, but several factors may influence their formation. Hormones Estrogen and progesterone are the hormones produced by the ovaries. They cause the uterine lining to regenerate during each menstrual cycle and may stimulate the growth of fibroids. Family history Fibroids may run in the family. If your mother, sister, or grandmother has a history of this condition, you may develop it as well. Pregnancy Pregnancy increases the production of estrogen and progesterone in your body. Fibroids may develop and grow rapidly while you’re pregnant.