OBJECTIVE To assess the clinical significance of umbilical cord pseudocysts detected prenatally by sonography. METHODS The prenatal sonographic findings, karyotype, and perinatal outcome in 13 fetuses with umbilical cord pseudocysts were reviewed retrospectively. RESULTS Umbilical cord pseudocysts were diagnosed at a median gestation of 27 weeks (range 15-37). Pseudocysts were single in eight cases with cyst diameters ranging from 20 to 50 mm, and double in one case. In the remaining four cases, multiple small cystic masses measuring less than 8 mm were identified. Additional sonographic findings were noted in 11 cases; ten of these fetuses had prenatal karyotyping, which showed trisomy 18 in five cases, trisomy 13 in one case, and a 46,XX, inv ins(18;21) complement in one case. Among the seven chromosomally abnormal fetuses, umbilical cord pseudocysts were multiple in four fetuses and single in three. All chromosomally abnormal fetuses and two euploid fetuses with associated structural defects died in utero or in the neonatal period. There were no perinatal complications in either of the fetuses with isolated pseudocysts. CONCLUSIONS The prenatal sonographic appearance of umbilical cord pseudocysts varied widely. These umbilical cord cystic masses were associated strongly with chromosomal disorders and structural defects, regardless of their sonographic appearance in utero.