Patent urachus is a type of urachal anomaly in which the urachus does not tail off but remains connected to the bladder in the umbilicus. The prevalence of patent urachus is very low. Herein, we report a case of patent urachus ruptured and exposed to amniotic fluid in utero. In this case, the size decreased after the second trimester, which was thought to be due to rupture in utero. After delivery, patent urachus was confirmed by inserting a foley catheter, which runs through a ruptured cyst on umbilical cord insertion. The day after delivery, the neonate underwent surgical excision of the urachal cyst and closing umbilicus. The mechanism of patent urachus rupture is unknown. As the fetus matures, it is thought that the higher intravesical pressure may affect the rupture of the cyst. Patent urachus could be ruptured in the uterus spontaneously, and surgical correction is needed. Therefore, prenatal differential diagnosis is important.
Keywords: patent urachus; urachal cyst; urachus.