Patent urachus is a rare congenital abnormality. Since its first description by Cabriolus in 1550, few cases have been reported. A 26-year-old Vietnamese primigravida presented at 20 weeks of gestation for evaluation of a cystic mass in the umbilical cord, which was first discovered at week 13 of pregnancy by ultrasound scan. The cystic mass originated from the root of the umbilical cord, connected to the urinary bladder, and no intestinal contents were enclosed within. Doppler ultrasound assessment showed that the single umbilical artery existed within the normal range. The progression of the umbilical cyst continued to be screened, but the mass disappeared on ultrasound images at 27 weeks of gestation. This led to the consideration of the cyst's rupture. After 38 gestational weeks, the pregnant woman delivered a 3350g male infant via cesarean section because of an obstructed vaginal labor. The following days, a stream of urine was recorded leaking out from the umbilical mass whenever he cried. Seven weeks after delivery, an open surgical approach was successfully performed. The baby is now 43 months of age, growing and developing normally. Since an allantoic cyst with patent urachus is a rare clinical entity, early discovery, close monitoring and accurate diagnosis through ultrasound in the prenatal period may consequently allow clinicians to have suitable attitudes towards management when the infant is born.