Level II Ultrasound:   The Fetal Abdomen





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Sonographic finding(s)

Differential Diagnosis


    Abdominal Masses

    Abdominal Wall Defects

    During the 6th week of development the abdominal cavity becomes too small to accommodate the growing intestine and the gut enters the umbilical cord producing physiologic umbilical herniation. The herniated loops of bowel normally return to the abdominal cavity by the 12th week.

    Source:University of New South Wales

    • Failure of the gut to return from the umbilical cord results in omphalocele.


      An omphalocele is the persistent herniation of the abdominal viscera into the umbilical cord. On ultrasound amnioperitoneal membrane is seen to cover an echogenic mass (herniated viscera) protruding through the umbilical ring. The umbilical cord inserts into the membrane. Polyhydramnios is often present. Occasionally the membrane surrounding the omphalocele will rupture making it difficult to distinguish an omphalocele from gastroschisis [1]. The prognosis is dependent on associated malformations and the size of the omphalocele (larger defects have a poorer prognosis).

      Omphalocele is seen in about 1 in 4,000 live births. Additional anomalies occur in up to 70% of cases [2, 3], and the defect is associated with several syndromes including pentalogy of Cantrell [4]and Beckwith-Wiedemann [5]. Chromosomal abnormalities, commonly trisomy 13 and 18 , are also found in up to 54% of affected fetuses [6].

      1. Cand LKH, et al, Prenatal rupture of omphalocele. J Clin Ultrasound 1987;15:191
      2. Hassan S, Hermansen MC:The prenatal diagnosis of ventral abdomina wall defects. Am J Obstet Gynecol 155:842,1986
      3. Sermer M, et al.;Prenatal diagnosis and managment of congenital defects of the anterior abdominal wall. Am J Obstet Gynecol 156:308,1987.
      4. Ghidini A, Sirtoni M, Romero R, et al: Prenatal diagnosis of pentalogy of Cantrell. J Ultrasound Med 7:567, 1988
      5. Koontz WL, Shaw LA, Lavery JP. Antenatal sonographic appearance of Beckwith-Wiedemann syndrome J Clin Ultrasound 1986;14:57
      6. Gilbert WM, Nicolaides KH: Fetal omphalocele: Associated malformations and chromosomal defects. Obtet Gynecol 70:633,1987


      Gastroschisis is a defect in the abdominal wall, usually to the right of the umbilicus, that allows bowel to protrude into the amniotic cavity. On ultrasound, multiple, round, freely floating structures along the abdominal wall suggests the diagnosis. The loops of bowel may be distinguished from loops of umbilical cord by the presence of blood flow in the latter on Doppler. The defect is thought to arise from disruption of blood flow (the right omphalomesenteric artery)to the affected area. [1]. Although gastroschisis is not associated with an increased rate of chromosomal abnormalities intestinal malrotation, atresia, volvulus,  and infarction may complicate the condition[2].The prognosis is generally favorable with survival rates between 77% and 100% [3,4]

      Gastroschisis occurs with an incidence of 1:10,000 births, and for unclear reasons is seen more frequently in younger mothers[5]. The defect is also seen more frequently in mothers who use vasoactive substances such as nicotine and cocaine [7,8]. The recurrence risk of 3% to 5% suggests a multifactorial mode of inheritance. Although, cases of a familial pattern of inheritance have been reported [6].

      Gastroschisis is associated with intrauterine fetal growth restriction , preterm delivery, and stillbirth [9,10]

      1. Hoyme HE, Higginbottom MC, Jones KL.  The vascular pathogenesis of gastroschisis: intrauterine interruption of the omphalomesenteric artery.  J Pediatr. 98:228, 1981
      2. Paidas MJ, Crombleholme TM, Robertson FM: Prenatal diagnosis and management of the fetus with an abdominal wall defect. Semin Perinatol 18:196, 1994
      3.Morrow RJ, Whittle MJ, McNay MB, Raine PA, Gibson AM, Crossley J. Prenatal diagnosis and management of anterior abdominal wall defects in the west of Scotland. Prenat Diagn 1993;13:111-5.
      4. Sipes SL, Weiner CP, Sipes DR, Grant SS, Williamson RA. Gastroschisis and omphalocele: does either antenatal diagnosis or route of delivery make a difference in perinatal outcome? Obstet Gynecol 1990;76:195-9.
      5. Calzolari E, Bianchi F, Dolk H, et al: Omphalocele and gastroschisis in Europe: A survey of 3 million births, 1980-1990. Am J Med Genet 58:187, 1995
      6. Torfs CP, Curry CJ: Familial cases of gastroschisis in a population-based registry. Am J Med Genet 45:465, 1993
      7.Haddow JE, Palomaki GE, Holman MS: Young maternal age and smoking during pregnancy as risk factors for gastroschisis. Teratology 47:225, 1993
      8. Hume RF, Gingras JL, Martin LS, et al: Ultrasound diagnosis of fetal anomalies associated with in utero cocaine exposure: Further support for cocaine-induced vascular disruption teratogenesis. Fetal Diagn Ther 9:239, 1994
      9. Tan KH, Kilby MD, Whittle MJ, et al: Congenital anterior abdominal wall defects in England and Wales 1987-1993: Retrospective analysis of OPCS data. BMJ 313:903, 1996
      10. Crawford RA, Ryan G, Wright VM, et al: The importance of serial biophysical assessment of fetal wellbeing in gastroschisis. Br J Obstet Gynaecol 99:899, 1992

    Ascites [1]
    • Meconium peritonitis

      • Obstructed bowel
      • Cystic fibrosis
      • Infections
    • Infection
      • Cytomegalovirus[1]
      • Parvovirus[1]
      • Syphilis[1]
      • Toxoplasmosis[2]
      • Hepatitis A[3,4]
      • Listeria[5]
    • Laryngeal atresia or stenosis[6,7]
    • Idiopathic[1]
    • Trisomy 21(Down syndrome)[1]
    • Turner syndrome
    • Genitourinary tract anomaly (Urinary ascites)[8,9]
    • Other metabolic or developmental [10-12]
    • Early Hydrops

    1. Zelop C, Benacerraf BR. The causes and natural history of fetal ascites. Prenat Diagn. 1994 Oct;14(10):941-6. PMID: 7899269
    2.Blaakaer J. Ultrasonic diagnosis of fetal ascites and toxoplasmosis. Acta Obstet Gynecol Scand. 1986;65(6):653-4.PMID: 3541487
    3. Leikin E, Lysikiewicz A, Garry D, Tejani N. Intrauterine transmission of hepatitis A virus. Obstet Gynecol. 1996 Oct;88(4 Pt 2):690-1.PMID: 8841254
    4. McDuffie RS Jr, Bader T.Fetal meconium peritonitis after maternal hepatitis A. Am J Obstet Gynecol. 1999 Apr;180(4):1031-2.PMID: 10203675
    5. Quinlivan JA, Newnham JP, Dickinson JE Ultrasound features of congenital listeriosis--a case report. Prenat Diagn. 1998 Oct;18(10):1075-8.PMID: 9826900
    6. Morrison PJ, Macphail S, Williams D, McCusker G, McKeever P, Wright C, Nevin NC. Laryngeal atresia or stenosis presenting as second-trimester fetal ascites--diagnosis and pathology in three independent cases. Prenat Diagn. 1998 Sep;18(9):963-7.PMID: 9793982
    7. Balci S, Altinok G, Ozaltin F, Aktas D, Niron EA, Onol B.Laryngeal atresia presenting as fetal ascites, olygohydramnios and lung appearance mimicking cystic adenomatoid malformation in a 25-week-old fetus with Fraser syndrome. Prenat Diagn. 1999 Sep;19(9):856-58.PMID: 10521845
    8. Bettelheim D, Pumberger W, Deutinger J, Bernaschek G.Prenatal diagnosis of fetal urinary ascites. Ultrasound Obstet Gynecol. 2000 Oct;16(5):473-5. PMID: 11169333
    9. Ohno Y, Koyama N, Tsuda M, Arii Y.Antenatal ultrasonographic appearance of a cloacal anomaly. Obstet Gynecol. 2000 Jun;95(6 Pt 2):1013-5. PMID: 10808008
    10. Schmider A, Henrich W, Reles A, Vogel M, Dudenhausen JW.Isolated fetal ascites caused by primary lymphangiectasia: a case report. Am J Obstet Gynecol. 2001 Jan;184(2):227-8.PMID: 11174507
    11. Lienhardt A, Aubard Y, Laroche C, Gilbert B, Bernard P, Massri K, Bouleisteix J, Masseri G.A rare cause of fetal ascites: A case report of Gunther's disease. Fetal Diagn Ther. 1999 Sep-Oct;14(5):257-61. Review.(congenital erythropoietic porphyria).PMID: 10529564
    12. Poulain P, Odent S, Maire I, Milon J, Proudhon JF, Jouan H, Le Marec B. Fetal ascites and oligohydramnios: prenatal diagnosis of a sialic acid storage disease (index case). Prenat Diagn. 1995 Sep;15(9):864-7.PMID: 8559759





    Umbilical Cord and Placenta


      Urinary Tract

      Around the fifth week of development the ureteric bud ( the future ureter and collecting system ) penetrates the metanephric blastema located in the embryonic sacrum and divides to form the renal pelvis. The ureteric bud continues to subdivide forming the collecting tubules which induce metanephric tissue to form excretory tubules (nephrons).

      Source:University of New South Wales

      • Premature division of the ureteric bud may lead to partial or complete duplication of the ureter.
      • Early degeneration of the ureteric bud or failure to induce the metanephric blastema to proliferate is believed to be the cause of renal agenesis.
      • Failure of the collecting tubules and excretory tubules to fuse results in the formation of cysts.

      Around the 35th day of gestation the ureter loses its lumen owing to the ascent of the kidney into the upper abdomen. The ureter recanalizes shortly after the kidney ascends.

      • Failure of the kidney to ascend results in a pelvic kidney.
      • Failure of either end of the ureter to recanalize may lead to ureteral obstruction.
        • In animal models early urinary tract obstruction in the second trimester results in renal dysplasia, whereas late obstruction in the third trimester results in hydronephrosis [1].

      Cause of a Dilated Urinary Tract [2,3]




      Pyelectasis refers to mild dilatation of the fetal renal pelvis. It has been defined by Corteville et al. as an anteroposterior renal pelvic diameter of:

      • 4 mm or greater before 33 weeks OR
      • 7 mm or greater after 33 weeks [4]

      While not an independent predictor of Down syndrome, pyelectasis has been found to be more common in fetuses with Down syndrome [2,3].

      The severity of isolated pyelectasis does not appear to increase the risk of aneuploidy [7]. However, the severity of pyelectasis may predict the development of hydronephrosis and possible postnatal complications. In a study by Mandell J et al.[8] fetuses determined to require postnatal surgery retrospectively had renal pelves measurements of

      • 5 mm or greater at 15 to 20 weeks
      • greater than 8 mm at 20 to 30 weeks
      • greater than 10 mm at 30 to 40 weeks
      • Search Literature

      Dilatation of the renal pelvis can be further characterized using the Society of Fetal Urology grading system.

      The Society of Fetal Urology (SFU) grading system for hydronephrosis:
      Grade 0--normal kidney with no hydronephrosis
      Grade 1--slightly dilated renal pelvis without caliectasis
      Grade 2--moderately dilated pelvis with mild caliectasis
      Grade 3--large renal pelvis, dilated calyces, and normal renal parenchyma
      Grade 4--very large renal pelvis, large dilated calyces, with thinning of the renal parenchyma

    1.Beck AD: The effect of intra-uterine urinary obstruction upon the development of the fetal kidney. J Urol 105:784, 1971
    2.Merguerian P: The evaluation of prenatally detected hydronephrosis. Monogr Urol 16(3):1-5, 1995
    3. Preston A, Lebowitz RL: What's new in pediatric uroradiology. Urol Radiol 11:217-220, 1989
    4.Corteville. Fetal Pyelectasis & Down Syndrome: Is Genetic Amniocentesis Warranted? Obstet Gynecol 1992; 79:770-2
    5.Whitlow BJ, Lazanakis ML, Kadir RA, Chatzipapas I, Economides DL. The significance of choroid plexus cysts, echogenic heart foci and renal pyelectasis in the first trimester. Ultrasound Obstet Gynecol 1998 Dec;12(6):385-90
    6.Vergani P, Locatelli A, Piccoli MG, Ceruti P, Mariani E, Pezzullo JC, Ghidini A Best second trimester sonographic markers for the detection of trisomy 21. J Ultrasound Med 1999 Jul;18(7):469-73
    7.Roshanfekr D, Vonpechman W, Petrikovsky B Severity of isolated fetal pyelectasis and risk of aneuploidy. Obstet Gynecol 2000 Apr 1;95(4 Suppl 1):S72
    8.Mandell J et al. Structural genitourinary defects detected in utero. Radiology. 1991 Jan;178(1):193-6.


    The bladder is routinely detected after 16 weeks. Absent in renal agenesis enlarged in obstructive uropathy.


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