From conception to delivery, a woman's uterus can grow from the size of a pear to the size of a watermelon. But pregnancy isn't the only potential reason for an enlarged uterus. An enlarged uterus is common and can be a symptom of a variety of medical conditions, some of which require treatment. Two of the most common causes of an enlarged uterus are uterine fibroids and adenomyosis. Uterine fibroids.