Hello - thanks for asking your question.
With unexplained ascities and an elevated CA-125 level, the obvious concern is for ovarian cancer. The sensitivity of the ultrasound for detecting ovarian cancer ranges from 60-100 percent, so the test is not totally conclusive. I would concur with the MRI as the next step to make sure that ovarian cancer is not present.
Without knowing the characteristics of the ascitic fluid (protein, serum-ascities albumin gradient, cell count etc.), it is impossible to say what exactly is causing the ascites. Causes can range from liver dysfunction, pancreatitis, congestive heart failure, infection, or kidney disease.
However, with the family history of cancer and the CA-125 level, I would make sure that ovarian cancer is not present.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.