We present a case of a large umbilical cord cyst detected at 21 weeks of gestation. Serial ultrasonographic examination revealed a single umbilical artery and progression of the cystic mass. A 2,842-g male infant was delivered at 37 weeks of gestation, and we confirmed that the umbilical cord cyst was a pseudocyst in our pathological examination. This case demonstrated an uneventful course of pregnancy despite the large umbilical cord pseudocyst.