A 40-year-old pregnant woman presented with a fetal abdominal cyst and oligohydramnios. Color Doppler scan revealed a single blood vessel from the fetal aorta into a single umbilical artery. Severe oligohydramnios limited ultrasonographic evaluation of the fetal lower limbs, kidneys, or bladder. The pregnancy was terminated; the fetus showed fused lower limbs, bulging abdomen, and absent external genitalia and was diagnosed with type III sirenomelia. On autopsy, no normal bladder was observed, but duodenal atresia, anorectal atresia, and right renal agenesis were found. An intra-abdominal cyst, diagnosed histologically as a saccular cloaca, occupied the abdominal cavity. Ultrasonographic diagnosis of fetal sirenomelia is difficult due to poor depiction of the lower limbs. A vitelline artery leading to a single umbilical artery and a fetal abdominal cyst occupying most of the abdominal cavity are considered fetal sirenomelia associated with large defects of the gastrointestinal and genitourinary tracts.