The allantois forms from the part of the fetal yolk sac that eventually becomes the primitive hindgut (the cloaca).

The cloaca divides into the hindgut posteriorly and the urogenital sinus anteriorly. The allantois remains connected to the urogenital sinus and extends into the base of the umbilical cord.

The umbilical cord is formed from the fusion of the body stalk (containing the allantois) and the umbilical vessels and yolk stalk (containing the omphalomesenteric duct) (1).

The allantois usually regresses by 14 menstrual weeks, but may occasionally persist and present as a cystic mass in the base of the umbilical cord (2).



  • Anechoic cystic mass of the umbilical cord.
  • Centrally located in the cord with the umbilical vessels located against the outer wall.
  • Located at the fetal end of the umbilical cord.
  • Size ranges from 5 mm to 5 cm (3,4).
  • May / may not communicate with the fetal bladder (5-7).
  • May be demonstrated as early as 9 menstrual weeks and may:
    • Decrease in size.
    • Resolve.
    • Persist throughout the pregnancy (3).


8 weeks of gestation




Video clip of Allantoic Cyst of the fetal end of the umbilical cord






  • Urinary tract anomalies (especially bladder outlet obstruction) in 18% of cases (8).
  • Omphaloceles (9).
  • Patent urachus (10).
  • Urachal cyst (10).




  1. Moore KL. The developing human. Second edition. Philadelphia: WB Saunders 1977;51, 61-62, 112-113, 205-209.
  2. Romero R, Pilu G, Jeanty P Prenatal diagnosis of congenital abnormalities. Norwalk: Appleton and Lange 1988;391-397.
  3. Rempen AR. Sonographic first-trimester diagnosis of umbilical cord cyst. J Clin Ultrasound 1989;17:53-55.
  4. Sachs L, Fourcroy JL, Wenzel DJ Prenatal detection of umbilical cord allantoic cyst. Radiology 1982;145:445-446.
  5. Hughes HA, Feinstein SJ, Lodeiro JG Umbilical cord cyst presenting as an omphalocele at 15 weeks gestation: a case report. J Reprod Med 1990;35:658-660.
  6. Donnenfeld AE, Mennuti MT, Templeton JM Prenatal sonographic diagnosis of a vesico-allantoic abdominal wall defect. J Ultrasound Med 1989;8:43-45.
  7. Harp J, Rouse GA, De Lange M. Sonographic prenatal diagnosis of an allantoic cyst. JDMS 1992;8:28-32.
  8. Herbst WP. Patent urachus. South Med J 1937;30:711-719.
  9. Fink IJ, Filly RA. Omphalocele associated with umbilical cord allantoic cyst: sonographic evaluation in utero. Radiology 1983;149:473-476.
  10. Yoo S -J, Lee Y -H, Joo MS Unusual fate of vesicoallantoic cyst with non-visualization of the fetal bladder in a case of patent urachus. Ultrasound Obstet Gynecol 1997;9:422-424.