RESUMEN
An urachus is a remnant of allantois in embryo development constituting a connection between the dome of the bladder and umbilicus. It develops on day 16 after conception and closes during the pregnancy to form the median umbilical ligament. Patent urachus results from a failure in closing its lumen in 10-12 gestational weeks. This anomaly occurs in 1-2 babies in 100,000 births. We present the case of patent urachus. In 20 gestational weeks, a dilatation of the umbilical cord with an anechoic mass with a transverse dimension of 19 x 12 mm starting from the fetal insertion and length of 30 cm was seen on ultrasound without any other fetal and placental disorders. Histology showed cystic edema. Prenatal diagnosis of patent urachus can be difficult because this pathology may be mistaken with other, more dangerous causes of cord cysts; thus, the occurrence of cord cysts should be closely monitored.