In the first trimester of embryonic life, the allantois communicates the dome of the anterior urinary component of the cloaca (later the urogenital sinus) to the umbilicus, allowing urine drainage. From the 12th to the 20th weeks of gestation, the lumen of the allantois involutes until becoming a fibromuscular remnant in the midline, the urachus, which connects the dome of the bladder, which has already become independent, to the umbilicus at the midline (Fig.1).
The urachus is variable in length (1-15 cm) and width (8-10 mm). The umbilicovesical fascia rests on its anterior surface and on that of the medial umbilical ligaments (fibrotic remnants of the umbilical arteries), delimiting the prevesical or Retzius extraperitoneal space (with a two-rooted molar shape) from the perivesical space, in which the urachus itself lies (Figs.2,3,4).
US (with linear, high-resolution probes if possible), CT (to be limited in pediatric and young patients, considering delayed excretory phase with the patient in prone position), MRI or fluoroscopy can be used to investigate urachal pathological conditions. On CT and MRI the urachus appears as a linear band running from the bladder dome to the umbilicus in the midline. It often goes unnoticed on US.