An isolated fetal ascites is a rare ultrasonographic finding. It is commonly diagnosed in association with fetal genitourinary or gastrointestinal conditions, mainly bowel obstruction. We present the sonographic features and neonatal outcome of a fetus with a large bowel obstruction, perforation and subsequent development of meconium peritonitis, prenatally diagnosed as isolated fetal ascites. A colonic atresia should be also included in the differential diagnosis of isolated fetal ascites.