Allantoic cyst: a prenatal clue to patent urachus.

A patent urachus, which is typically diagnosed as leakage from the umbilicus postnatally, can present as an allantoic cyst in the umbilical cord antenatally. We report a case of a patent urachus with an allantoic cyst diagnosed via fetal MR imaging at 24 weeks' gestation. Early detection allowed for appropriate counseling and prompt corrective surgery after birth.