During this complex transformation of the IVC, numerous variations and anomalies to its adult form may occur. The most frequent is azygos continuation of the IVC, which has also been termed absence of the hepatic segment of the IVC with azygos continuation . Failure to form the right subcardinal-hepatic anastomosis (with resulting atrophy of the right subcardinal vein) determines that the anastomosis between the subcardinal and right supracardinal veins collects the blood from the lower body, diverting it to the cranial end of the right supracardinal vein (azygos vein). This passes posterior to the diaphragmatic crura to enter the thorax and joins the superior vena cava at the normal location in the right paratracheal space [7,8]. The hepatic segment is ordinarily not truly absent; rather, it drains directly into the right atrium.
Under normal conditions, the only major vessel that can be observed behind the heart is the descending aorta, which is positioned on the left side of the spine and on the same side as the cardiac apex. The finding of two vessels running behind the heart in the four-chamber view is usually pathological and may be caused by interrupted IVC with azygos continuation (excellent marker of left isomerism) or by total anomalous pulmonary venous connection (typical of right isomerism) . In the first case, the aorta and azygos vein are located in close proximity on the same side of the spine (“double vessel” sign), whereas in the second, the pulmonary venous confluence is situated immediately behind the atrium and a wide gap is apparent between the posterior wall of the atrium and the descending aorta.
Interruption of the IVC with azygos continuation is diagnosed by imaging two vessels (“double vessel” sign) of similar size in a paraspinous location posterior to the heart . This sign is easily detected in transverse views of the thorax and abdomen, and can be confirmed on longitudinal views. Compared to the normal relationship, which shows only the aorta posterior to the heart, interruption of the IVC results in collateral flow through the azygos vein, which becomes enlarged and readily visible. This should be distinguished from the normal azygos vein, which occasionally can be visualized in the third trimester as a small 1 to 2 mm structure.
The “double vessel” sign has been described in a high percentage (80 to 96%) of left isomerism [9,10,11], but it can also be found in few cases of right isomerism  and as a benign vascular malformation with situs solitus and without congenital heart disease [1,2,3,9,13]. In combination with cardiac anomalies or situs abnormalities, interruption of the IVC with azygos continuation should suggest a specific diagnosis of heterotaxy, especially left isomerism. In addition, a strong association between large omphaloceles and interruption of the IVC with azygos continuation has also been documented prenatally .
In any case of diagnosis of an interruption of the IVC, a thorough fetal search for associated anomalies is indicated, with a special emphasis on possible cardiac abnormalities and heterotaxy. It is convenient to inform parents of the good prognosis if it is an isolated finding, although it is important to inform them of possible complications associated with invasive procedures and of the increased risk of deep-vein thrombosis.
 Savirón Cornudella R, Pérez Pérez P, De Diego Allué E, Cisneros Gimeno A, Corona Bellostas C, Castán Mateo S. Interrupción de vena cava inferior. Diagnóstico prenatal de sus variantes. Rev chil obstet ginecol. 2017;82(6):626-632. https://dx.doi.org/10.4067/S0717-75262017000600626
 Celentano C, Malinger G, Rotmensch S, Gerboni S, Wolman Y, Glezerman M. Prenatal diagnosis of interrupted as an isolated finding: a benign vascular malformation. Ultrasound Obstet Gynecol. 1999;14(3):215‐218. doi:10.1046/j.1469-0705.1999.14030215.x
 Bronshtein M, Khatib N, Blumenfeld Z. Prenatal diagnosis and outcome of isolated interrupted inferior vena cava. Am J Obstet Gynecol. 2010;202(4):398.e1‐398.e3984. doi:10.1016/j.ajog.2009.11.014
 Ginaldi S, Chuang VP, Wallace S. Absence of hepatic segment of the inferior vena cava with azygous continuation. J Comput Assist Tomogr. 1980;4(1):112‐114. doi:10.1097/00004728-198002000-00021
 Sheley RC, Nyberg DA, Kapur R. Azygous continuation of the interrupted inferior vena cava: a clue to prenatal diagnosis of the cardiosplenic syndromes. J Ultrasound Med. 1995;14(5):381‐387. doi:10.7863/jum.19220.127.116.111
 Ruggeri M, Tosetto A, Castaman G, Rodeghiero F. Congenital absence of the inferior vena cava: a rare risk factor for idiopathic deep-vein thrombosis. Lancet. 2001;357(9254):441. doi:10.1016/S0140-6736(00)04010-1
 Bass JE, Redwine MD, Kramer LA, Huynh PT, Harris JH Jr. Spectrum of congenital anomalies of the inferior vena cava: cross-sectional imaging findings. Radiographics. 2000;20(3):639‐652. doi:10.1148/radiographics.20.3.g00ma09639
 Fernando Viñals L, Marcela Muñoz F, Arrigo Giuliano B. (2002). Marcadores sonográficos de cardiopatías congénitas. Interrupción de la vena cava Inferior--a propósito de nuestra experiencia y resultados. Rev chil obstet ginecol. 2002. 67(4):280-287. https://dx.doi.org/10.4067/S0717-75262002000400003
 Berg C, Georgiadis M, Geipel A, Gembruch U. The area behind the heart in the four-chamber view and the quest for congenital heart defects. Ultrasound Obstet Gynecol. 2007;30(5):721‐727. doi:10.1002/uog.5152
 Thakur V, Jaeggi ET, and Yoo SJ. "Abnormal visceral and atrial situs and congenital heart disease." Fetal Cardiology: Embryology, Genetics, Physiology, Echocardiographic Evaluation, Diagnosis and Perinatal Management of Cardiac Diseases, 3rd edition. Boca Raton, FL: CRC Press, 2019: pgs 239-252.
 Berg C, Geipel A, Kamil D, et al. The syndrome of left isomerism: sonographic findings and outcome in prenatally diagnosed cases. J Ultrasound Med. 2005;24(7):921‐931. doi:10.7863/jum.2005.24.7.921
 Ruscazio M, Van Praagh S, Marrass AR, Catani G, Iliceto S, Van Praagh R. Interrupted inferior vena cava in asplenia syndrome and a review of the hereditary patterns of visceral situs abnormalities. Am J Cardiol. 1998;81(1):111‐116. doi:10.1016/s0002-9149(97)00811-4
 Giang do TC, Rajeesh G, Vaidyanathan B. Prenatal diagnosis of isolated interrupted inferior vena cava with azygos continuation to superior vena cava. Ann Pediatr Cardiol. 2014;7(1):49‐51. doi:10.4103/0974-2069.126558
 Mlczoch E, Carvalho JS. Interrupted inferior vena cava in fetuses with omphalocele. Case series of fetuses referred for fetal echocardiography and review of the literature. Early Hum Dev. 2015;91(1):1‐6. doi:10.1016/j.earlhumdev.2014.11.001