Why does abdomen fill up with fluid when ovarian cancer is present?

Not always the case: Ovarian cancer can cause inflammation which can result in fluid accumulation in the space surrounding the intestinal tract called the peritoneum. Some types of ovarian cancer make mucous which can also fill this space. Both situations cause fluid accumulation, weight gain and bloating. However, ovarian cancer may not cause these symptoms which is why regular checkups are important.

Protein: Ovarian cancer can block lymph vessels, cause inflammation, obstruct the liver, and/or create leaky veins. All of this leads to protein-rich fluid seeping into the abdomen, and that protein-rich fluid (called ascites) draws more fluid toward it. Remember diffusion and osmosis labs in high school? Other cancers involving the liver/abdomen (breast, colon, etc.) can cause the same fluid build-up.