Primary ovarian leiomyoma is a rare type of benign neoplasm. Ovarian leiomyoma cases until a recent date usually showed favorable pregnancy outcomes. Contrary to others, the present authors report a case of a nine-week miscarriage with a giant primary ovarian leiomyoma. This neoplasm originated from the ovary with estrogen secreted by the endocrine organs. Hormone secretion is increased during pregnancy, particularly in the first trimester; it is supposed that it stimulated growth and progression of the mass. A close examination in adnexa is necessary at prenatal check.