Persistent patent urachus with allantoic cyst: a case report.

Patent urachus results when there is a persistence of an allantois remnant which normally undergoes atresia during embryological development. It can lead to an abdominal wall defect similar in appearance on ultrasound to an omphalocele. A 34-year-old primigravida presented at 19 weeks' gestation for evaluation of a cystic mass arising at the umbilical cord insertion. The initial impression of the referring physician was an omphalocele. The mass arose from the abdominal wall and the umbilical cord inserted into the membranous covering of the mass, which appeared to be fluid-filled and separate from but contiguous with the urinary bladder. Serial sonography followed the progression of the abdominal wall mass. At term, the patient underwent primary Cesarean section with delivery of a 4494-g male infant. The infant underwent repair and closure of the patent urachus and plastic reconstruction of the abdominal wall. When the urachus remains patent, it can lead to a urinary fistula which mimics the ultrasound appearance of an omphalocele. However, patent urachus is associated with a much lower rate of abnormalities than omphalocele, yielding a better fetal prognosis.