Benign Ovarian Cysts: The presence of clear septated cysts that are stable or newly developed, combined with relatively low and stable CA125 levels, suggests a benign nature. The fact that the first cyst remained unchanged and the second cyst is also clear septated supports this diagnosis.

Simple Ovarian Cysts: These are common in postmenopausal women and can be multiple. The description of the cysts as clear septated could also align with simple cysts, especially given the lack of significant change in CA125 levels.
Serous Cystadenomas: These are benign ovarian tumors that can present as septated cysts. They are common in postmenopausal women and would be consistent with the imaging findings.

Ovarian Cancer (Epithelial): Although the stable nature of the first cyst and the low CA125 levels are reassuring, ovarian cancer cannot be entirely ruled out without further investigation. The development of a new cyst, even if it appears benign, warrants careful consideration of malignancy, especially in a postmenopausal woman.
Borderline Ovarian Tumors: These tumors have a lower malignant potential than ovarian carcinomas but can still pose significant risk. Their presentation can be similar to benign cysts, making them a crucial differential diagnosis.

Mucinous Cystadenomas: While these are benign, they can present as large, septated cysts. However, they are less common than serous cystadenomas and simple cysts.
Dermoid Cysts (Mature Cystic Teratomas): These can occur at any age, including postmenopause, but are less likely given the description of clear septated cysts, which does not typically align with the characteristic features of dermoid cysts.