Case 48 Answer
Statistically, the most likely diagnosis of a well-defined hypoechoic mass with thin septations and a thin echogenic rim is a fibroadenoma. However, with the history of a rapidly enlarging palpable mass during pregnancy, this most likely represents (and was found to be) a lactating adenoma.
Lactating adenomas tend to be large well-defined lobulated masses with fibrotic septae which can appear as echogenic bands within the mass. They are benign, but are usually removed surgically because of their large size and rapid growth.