Aneurysm of the umbilical cord is an extremely rare vascular anomaly. We report a case of umbilical cord aneurysm with arteriovenous fistula in a fetus with trisomy 18. At 34 weeks' gestation a fetus with suspected intrauterine growth restriction and oligohydramnios was referred to our institution. Ultrasound examination was confirmatory and additionally revealed multiple markers for trisomy 18. In the umbilical cord an aneurysm was diagnosed characterized by a cystic lesion with hyperechogenic rim measuring 18 x 18 x 19 mm in diameter. Color flow and spectral Doppler examinations showed a jet originating from one of the umbilical arteries entering the cystic lesion which appeared to be the dilated umbilical vein. Fetal blood sampling and subsequent karyotyping revealed trisomy 18 (47, XY, +18). The patient elected to terminate the pregnancy. Pathologic examination of placenta and umbilical cord confirmed the prenatal diagnosis of umbilical cord aneurysm and arteriovenous fistula. Histology demonstrated a strongly dilated umbilical vein, one moderately dilated artery and a second, apparently normal artery.