This is a sonographically inconclusive lesion, based on IOTA simple rules criteria 2. The presence of mild (IOTA color score 2) internal vascularity and the size of the multilocular mass put this outside the "likely benign" range, however none of the IOTA malignant features are present.
Likely differential is between a cystadenoma or borderline epithelial cystic neoplasm, with mucinous favored over serous based on the presence of internal septations.
Path-proven borderline mucinous cystadenoma.