BackgroundDuring pregnancy, both ovarian hyperstimulation syndrome (OHSS) and pregnancy luteoma could manifest as massive ascites, enlarged ovaries, or elevated serum levels of cancer antigen 125 (CA125), and atypical cells may be found in the ascitic fluid of OHSS patients. Whether this should be treated aggressively as peritoneal carcinomatosis is controversial.Case presentationA 35-year-old G2P1A1 woman with secondary infertility had a successful pregnancy after one cycle of assisted reproductive technology. The patient complained of lower abdominal distension, oliguria, and poor appetite 19 days after embryo transplantation. She was diagnosed with late-onset OHSS. Although the size of the ovaries decreased bilaterally to the normal range at 12 weeks of gestation after prompt medical care, the ascites increased again after an initial decreasing trend. Elevated serum levels of CA125 (191.1 IU/mL), and suspected adenocarcinoma cells were observed in the ascitic fluid. Although further magnetic resonance imaging examination or diagnostic laparoscopy was recommended, the patient was provided with supportive treatment and closely monitored upon her request. Surprisingly, her ascites diminished, and serum level of CA125 started to decline at 19 weeks of gestation. During cesarean section, pathological examination of the solid mass in the right ovary revealed pregnancy luteoma, which was presumably the other cause of the intractable ascites.ConclusionsCaution should be exercised in cases of suspicious malignant ascites during pregnancy. This may due to OHSS or pregnancy luteoma, in which abnormalities usually regress spontaneously.