Lectures/Topics

Gastroschisis

Gastroschisis is a paraumbilical defect involving all the layers of the abdominal wall. It is usually right-sided with the small bowel herniated through the defect. The stomach or other organs may occasionally be involved in herniation.

Fig 1

Incidence: 1 in 2000-5000 live births with a high prevalence among the maternal age of 11-19 years. It has been associated with various medications including recreational drugs, NSAIDs (aspirin, salicylate, ibuprofen), and decongestants (pseudoephedrine and phenylpropanolamine) . There has been a sustained increase in the incidence over the past decade, particularly in teenage women.

Sonographic findings:

• Abdominal organs herniate through an anterior abdominal wall defect; typically, there are multiple loops of bowel outside the abdomen. Fig2, Fig3, Fig4
• No covering membrane.
• Umbilical cord inserting on the abdominal wall (normal insertion site). Fig5, Fig6
• Varying degrees of bowel dilatation and wall thickening or increasing intraluminal meconium. Fig7, Fig8, Fig9
• The liver is very rarely involved. Fig10
• The amniotic fluid volume is usually normal, however, oligohydramnios may be observed in 36% of cases. Polyhydramnios may be seen in the case of bowel obstruction.
• An increase in serum alpha-fetoprotein levels, higher than those associated with omphalocele.
• Doppler velocity of the superior mesenteric artery found to be predictive of outcome in fetuses with gastroschisis.
• Usually first diagnosable in the early second trimester; the diagnosis prior to 13 weeks may be confused with physiologic herniation.
• Pitfalls: The cases with oligohydramnios, particularly in late pregnancy, can easily be missed. Additionally, extra-abdominal bowel can be overlooked and thought to be a coiled umbilical cord.
• Differential diagnoses include other types of abdominal wall defects (see omphalocele), especially ruptured omphalocele, though rare, limb-body wall complex, and tangled cord adjacent to the fetal abdomen, in which color flow will show vascular flow.
• Most standard ultrasound parameters are not significantly associated with an adverse neonatal outcome, except for polyhydramnios, which was strongly predictive of severe bowel complications in the neonatal period.

Fig 1:  Schematic drawing of gastroschisis; note normal location of the cord insertion

Fig 2:  Gastroschisis   Scan of the free-floating bowel loops in the amniotic fluid

Fig 3:  Gastroschisis  Cross-sectional scan of the abdomen: Free floating dilated bowel loops (arrow) in the amniotic fluid (arrowhead = spine, * = intra-abdominal stomach)

Fig 4:  Gastroschisis  Cross-sectional scan of the abdomen: bowel (arrowhead) protruding through the abdominal wall defect on the right-side of the umbilicus (arrow = umbilical vein)

Fig 5:  Gastroschisis  Cross-sectional scan of the abdomen: free floating bowel in the amniotic fluid on the right side of the umbilical vessels (*) protruding through the abdominal wall defect

Fig 6:  Gastroschisis   Cross-sectional scan of the abdomen: Free floating bowel loops (arrow) in the amniotic fluid, protruding through the abdominal wall defect on the right-side of the umbilical vein (*)  (solid circle = intra-abdominal stomach)

Fig 7:  Gastroschisis  Free floating dilated bowel loops in the amniotic fluid

Fig 8:  Gastroschisis   Free floating dilated bowel loops (*) in the amniotic fluid

Fig 9:  Gastroschisis  Free floating bowel loops (*), thickened wall caramelized by the turbid amniotic fluid

Fig 10:  Gastroschisis   Cross-sectional scan of the abdomen: liver mass (arrow) and bowel (arrowhead) without covering membrane protruding through large defect, note normal cord insertion (*) (solid circle = spine)

Associations: Related to other anomalies in 7-10% of cases, mostly GI anomalies, not associated with chromosomal abnormalities.

Management: Serial sonographic monitoring is necessary to evaluate the degree of dilation and wall thickness. Early delivery may be beneficial in cases of marked dilatation or thickened bowel wall, however, term delivery gives the best outcomes in most cases. Delivery should be performed in a tertiary center with appropriate facilities for surgical management of the newborn. The fetuses may carry a higher rate of fetal distress and intrauterine death and fetal surveillance is recommended, however, labor and ruptured membranes do not appear to be associated with increased neonatal morbidity or mortality rates in neonates with gastroschisis. Cesarean section probably does not improve the outcomes.

Prognosis: Good but complications are very common, especially those related to the gastrointestinal tract problems and prematurity, increased morbidity in cases of bowel obstruction, bowel wall thickening, or increased amniotic concentration of digestive compounds.

Recurrence risk: Sporadic except for rare familial cases.

Omphalocele

Omphalocele is a defect in the anterior abdominal wall with extrusion of abdominal contents in the base of the umbilical cord. The herniated mass is covered by parietal peritoneum and amnion, with Wharton’s jelly intervening between the two membrane layers.

Fig 1, Fig 2

Incidence: 1 in 3000-4000 births with a male to female ratio of about 1:5. An increased incidence has been observed with advancing maternal age.

Sonographic findings:

• Typically, a midline mass with a covering membrane adjacent to the abdominal wall containing abdominal contents (bowel or liver). Eighty percent of cases contain liver, sometimes with small bowel. The stomach and bladder may occasionally lie in the omphalocele. Fig 3, Fig 4, Fig 5, Fig 6, Fig 7
• Variable depending on the amount of ascites, degree of herniation and associated anomalies.
• Twenty percent contain gut and fluid only and most abnormal chromosomes are seen in this subgroup.
• Omphaloceles containing bowel alone tend to be small and can be missed or mistaken for gastroschisis. Fig 8
• Umbilical cord inserting on the sac.
• Ascites commonly seen.
• The covering membrane may not always be seen, particularly in the absence of ascites or when ruptured.
• Associated malformation commonly seen.
• Usually first diagnosable in the second trimester but can be detected in the first trimester if the liver is present within the mass, especially using three-dimensional ultrasound. In physiologic omphalocele, only the gut will be seen in the herniated sac.
• An anterior abdominal wall mass >7 mm at any crown-rump length (CRL), or of any size in a fetus of CRL >44 mm, is suggestive of a fetal anomaly. Alternatively, a cord base mass within the 4-7 mm range for a CRL of 19-44 mm can be considered normal and not to require any follow-up.
• Three-dimensional ultrasound can be useful for additional information and more efficient counseling and postnatal therapeutic planning.
• A false-positive diagnosis of omphalocele, or pseudo-omphalocele, can be produced by scanning in an oblique plane or by compressing the fetal abdomen.
• Main differential diagnoses include:
  • Gastroschisis: no surrounding membrane, normal cord insertion
  • Umbilical hernia: covered by skin rather than a membrane, indistinguishable from a small omphalocele
  • Bladder/cloacal exstrophy: a mass below the cord insertion site with no visible bladder
  • Limb-body wall complex: the placenta attached to the fetus, absent or very shortened cord, limbs and spine defects
  • Pentalogy of Cantrell: ectopia cordis with omphalocele
  • Urachal and omphalomesenteric cyst: cysts at the cord insertion site, no visceral contents

Fig 1:  Schematic drawing of physiologic omphalocele at 8 weeks of gestation with the disappearance at 12 weeks

Fig 2:  Schematic drawing of omphalocele containing liver and bowel; note the cord insertion on the covering membranes

Fig 3:  Omphalocele  Free floating sac (arrow) containing liver and bowel (*) with covering membrane (arrowhead) (arrow = omentum)

Fig 4:  Omphalocele  Free floating liver mass with covering membrane (*) in amniotic fluid

Fig 5:  Omphalocele  Cross-sectional scan of the abdomen: liver mass (*) protruding through large defect, invisible covering membrane blending with the liver (solid circle = spine)

Fig 6:  Small omphalocele  Cross-sectional scan of the abdomen: small part of the liver (solid circle) protruding through the umbilicus, (* = liver, arrow = umbilical vein)

Fig 7:  Omphalocele  Liver mass (solid circle) with covering membrane (arrows) located outside of the body

Fig 8:  Small omphalocele  Cross-sectional scan of the abdomen: bowel (arrow) protruding through the umbilicus (* = liver, arrowhead = spine)

Associations: Mostly sporadic but associated with other anomalies in 50-85% of cases (cardiac defect in 45-50%, and other anomalies such as neural tube defects, diaphragmatic hernia, etc.), and chromosomal abnormalities in 30% of cases, especially trisomy 18 and 13, particularly when the sac contains no liver and is associated with another anomaly.

Management: A careful search for associated anomalies and karyotyping are indicated. For isolated omphalocele, delivery should occur in a tertiary center with appropriate facilities for surgical management of the newborn. A multidisciplinary approach to the fetus can improve the neonatal outcome. There is still a relatively high rate of elective termination of pregnancies for both defects, even in isolated cases which generally have a good prognosis after surgical repair. Cesarean section probably does not improve the outcomes.

Prognosis: Good for isolated lesions with proper management but poor when associated with other anomalies or chromosome abnormalities.

Recurrence risk: Sporadic for isolated cases, but there is probably a high recurrent rate in cases with rare autosomal dominant or X-linked recessive inheritance.

Abnormal Genitalia

Ambiguous genitalia is the term used when on examination there is doubt as to whether the patient is phenotypically male or female. Ambiguous genitalia are made up of a heterogeneous group of disorders of various etiologies that are usually related to either an abnormal hormonal influence such as congenital adrenal hyperplasia or chromosomal defects such as trisomy 13 or trisomy 22.

Sonographic findings:

The types of abnormalities that lead to a suspicion of abnormal genitalia are short phallus, bent phallus, bifid scrotum, and apparently normal female genitalia in a fetus confirmed to be a karyotypic male, either by chorionic villous samplings or amniocentesis. If an XY karyotype is confirmed but a careful sonographic evaluation demonstrates normal-appearing female external genitalia, then the diagnosis of testicular feminization (XY chromosomes with female external genitalia) is assumed.

The short or bent penis seen in association with a normal-appearing scrotum is usually an indication of hypospadias with chordae.

When pseudohermaphroditism was detected in a male fetus by an experienced ultrasonographer at a tertiary center the prenatal diagnosis was accurate in 100% of cases. The prenatal diagnosis was less accurate (46% correct) in a female fetus.

Rarely, an examiner may perceive the genitalia as labia with a large clitoris or a bifid scrotum with a short penis. In such situations, a diagnosis of ambiguous genitalia may be made. However, the accurate diagnosis of ambiguous genitalia can be extremely difficult and should not place a diagnostic label on a fetus prenatally.

Autosomal Recessive Polycystic Kidney

Autosomal recessive polycystic kidney (PCK; Potter type I) is an autosomal recessive disorder, previously termed infantile PCK, characterized by bilateral microcystic change of renal tubules leading to enlarged non-functional kidneys with maintenance of their reniform shape. Most cases manifest in utero but some may be evident after birth or childhood. PCK may be associated with abnormality of fibrocystin, a receptor protein that acts in collecting-duct and biliary differentiation.

          Fig 1

Incidence: 1 in 40,000-50,000 births, male/female, 1:1.

Sonographic findings:

• Bilateral enlarged (above the 90th percentile for length and width), echogenic kidneys with reniform shape. Fig 2, Fig 3, Fig 4, Fig 5
• Oligohydramnios with non-visualization of the bladder, usually not before 15 weeks.
• Pulmonary hypoplasia.
• Usually diagnosed after 18 weeks.

Fig 1:  Schematic drawing of polycystic kidney (enlarged reniform shape)

Fig 2:  Polycystic kidneys   Coronal scan of the abdomen: symmetrical markedly enlarged echogenic kidneys (* = renal pelvis)

Fig 3:  Polycystic kidneys   Cross-sectional scan of the abdomen: symmetrical markedly enlarged echogenic kidneys (* = renal pelvis, arrowhead = spine)

Fig 4:  Polycystic kidneys  Coronal scan of the abdomen: symmetrical markedly enlarged echogenic kidneys

Fig 5:  Polycystic kidneys   Oblique coronal scan of the abdomen: symmetrical enlarged echogenic kidneys containing numerous microcystic lesions (* = renal pelvis)

Associations: Hepatic fibrosis, Meckel Gruber syndrome, trisomy 13.

Management: Termination of pregnancy can be offered. Vaginal delivery without electronic fetal monitoring in labor is appropriate.

Prognosis: Very poor, particularly the cases with severe oligohydramnios and fetal kidneys >4 SD.

Recurrence risk: About 25%.

Multicystic Dysplastic Kidney (MCDK)

MCDK is a congenital renal dysplasia characterized by multiple, smooth-walled, non-functioning, non-communicating cysts of variable size and number, replacing all or nearly all normal parenchyma. Normal nephrogenesis may occur before the secondary changes leading to MCDK.

Fig 1

Incidence: 1 in 3000 live births, more common in boys.

Sonographic findings:

• Enlarged kidney with multiple non-communicating cysts of varying size. Fig2, Fig3, Fig4, Fig5, Fig6
• The lesion is unilateral in 70-80% of cases but 20-30% have a renal anomaly of the contralateral kidney such as hydronephrosis or renal agenesis.
• Normal amniotic fluid volume if unilateral and oligohydramnios if bilateral.
• Differentiation from severe hydronephrosis (with several dilated calyces) may be difficult in some cases.
  • Severe hydronephrosis: the oval cysts communicate with each other and the renal pelvis; renal parenchyma is usually visualized, and ureteral dilatation is often seen.
  • MCDK: multiple round cysts of varying size are non-communicating with each other or the renal pelvis, renal parenchyma is often not seen, and ureteral dilatation is not visualized.

Fig 1:  Schematic drawing of multicystic kidney

Fig 2:  Small multicystic kidney  Oblique cross-sectional scan of the abdomen: multiple cysts varying in size in the kidney (arrow)

Fig 3:  Multicystic kidney  Sagittal scan of the abdomen: multiple cysts varying in size in the kidney (*)

Fig 4:  Multicystic kidney  Sagittal scan of the abdomen: multiple cysts varying in size in the kidney (*)

Fig 5:  Multicystic kidney  Cross-sectional scan of the abdomen: multiple cysts varying in size in the kidney

Fig 6:  Multicystic kidney  Cross-sectional scan of the abdomen: multiple cysts varying in size in the kidney (arrow = spine)

Associations: Renal anomalies of the contralateral kidneys (about 40%) and abnormalities of the lower urinary tract are common.

Management: Unilateral MCDK should not alter the standard obstetric care. For bilateral non-functioning MCDK with oligohydramnios, termination of pregnancy should be considered.

Prognosis: Good, depending on the contralateral kidney and associated anomalies; recurrent risk in subsequent pregnancy is low (<5%).

Recurrence risk: Sporadic.

Bladder Outlet Obstruction

Bladder outlet obstruction occurs almost exclusively in males, and is most often related to the posterior urethral valve, resulting in failure of complete disintegration of the urogenital membrane leaving membranous tissue within the posterior urethra; it is rarely secondary to a nearby lesion such as cloacal dysgenesis or ureterocele. It is the most common cause of severe obstructive uropathy in childhood.

Incidence: 1 in 5000-8000 boys.

Sonographic findings:

• Only males are affected.
• The urinary bladder is dilated (megacystis) regardless of the cause. Fig 1, Fig 2
• The bladder wall is commonly thickened.
• The posterior urethra may be dilated and appears as a projection from the bladder base, giving a keyhole appearance. Fig 3, Fig 4
• The ureters are usually dilated (megaureter). Fig 5, Fig 6, Fig 7
• Asymmetric hydronephrosis, with massive hydronephrosis on one side and minimal hydronephrosis on the other with relative sparing.
• The kidneys have a variable appearance depending on the presence and extent of dysplasia including hydronephrosis, cystic renal parenchyma, or small shrunken kidneys with echogenic parenchyma.
• Oligohydramnios with pulmonary hypoplasia.
• Prolonged distention of the bladder with deficiency of the abdominal musculature development, called Prune belly syndrome, is commonly seen.
• Usually presenting at 18-22 weeks but possibly as early as 11 weeks.

Fig 1:  Megacystis   Sagittal scan of the trunk: marked dilatation of the bladder (solid circle) with protrusion of the lower abdomen (arrow = diaphragm)

Fig 2:  Megacystis   Sagittal scan of the trunk: marked dilatation of the bladder (*), occupying nearly total abdomen

Fig 3:  Megacystis   Coronal scan of the lower abdomen: dilatation of the bladder (solid circle) with marked dilatation of the proximal urethra giving the key-hole appearance (*)

Fig 4:  Megacystis   Coronal scan of the lower abdomen: dilatation of the bladder (solid circle) with thickened wall; dilatation of the proximal urethra giving the key-hole appearance (*)

Fig 5:  Megacystis   Cross-sectional scan of the abdomen: marked dilatation of the bladder (solid circle) and ureter (+) and renal pelvis (*) with echogenic thin parenchyma (arrow = spine)

Fig 6:  Megaureter and megacystis  Oblique scan of the abdomen: marked dilatation of the ureter (arrow), simulating bowel loop, and bladder (*) (arrowhead = spine)

Fig 7:  Megacystis and megaureter  Coronal scan of the abdomen: marked dilatation of the ureter (arrow) with dilated renal pelvis and dilated bladder

Associations: Most are isolated and are rarely related to urethral atresia or caudal regression, and aneuploidy (trisomy 21, 18 and 13) in some cases.

Management: Termination of pregnancy can be offered when diagnosed before viability when oligohydramnios or renal insufficiency is present. In utero decompression can be considered in some selected cases. Cytogenetic analysis or FISH can be done using fetal urine specimens. The cause of obstruction may be corrected in some cases, such as ureterocele.

Prognosis: Variable, but most are poor, especially when combined with severe oligohydramnios, lung hypoplasia and renal dysplasia.

Recurrence risk: Sporadic (rare recurrence) with rare familial inheritance.

Hydronephrosis

Hydronephrosis is the dilatation of the fetal renal collecting system. It is the most common fetal abnormality detected by antenatal ultrasound. It can result from ureteropelvic junction (UPJ, 41%), ureterovesical junction (UVJ, 23%), duplication of the collecting system (13%), and bladder outlet (10%). Renal dysplasia can occur in some fetuses with severe or early-onset obstruction.

UPJ obstruction is a functional obstruction and is bilateral in 30% of cases. It affects males more than twice as often as females. Most cases do not become dysplastic.

Vesicoureteral reflux is much more common among males and is often bilateral. It often resolves within the first years of life. Reflux that does not resolve spontaneously or is very severe at birth can be corrected surgically.

Incidence: 1-5 per 1000 births (ureteropelvic junction obstruction: male/female, 4:1, ureterovesicular junction: male>female).

Sonographic findings:

• A diagnosis of hydronephrosis should be made when there is calicectasis or the antero-posterior diameter of the renal pelvis, measured on a transverse view through the kidney, is at least 8 mm at 16-20 weeks or at least 10 mm after 20 weeks of gestation. Fig 1, Fig 2, Fig 3, Fig 4, Fig 5 Fig 6, Fig 7

Fig 1:  Hydronephrosis  Coronal scan of the posterior abdomen: mild dilated renal pelvis (*) (arrow = bifurcation of aorta)

Fig 2:  Hydronephrosis  Coronal scan of the abdomen: bilateral markedly dilated renal pelvis (*) (arrow = aorta)

Fig 3:  Hydronephrosis  Sagittal scan of the abdomen: markedly dilated renal pelvis (*) and calyx (arrow)

Fig 4:  Hydronephrosis   Cross-sectional scan of the abdomen: markedly dilated renal pelvis (*) and calyx (arrow) (arrowhead = spine)

Fig 5:  Hydronephrosis   Cross-sectional scan of the abdomen: markedly dilated renal pelvis (*) with calyces (arrow)

Fig 6:  Hydronephrosis  Parasagittal scan of the abdomen: markedly enlarged renal pelvis (*) and calyces (arrow)

Fig 7:  Severe hydronephrosis  Sagittal scan of the abdomen: markedly enlarged renal pelvis (*) with echogenic thin parenchyma

• The Society for Fetal Urology recommends a scoring system to grade dilation of the upper urinary tract as follows:
  • Grade 0: no dilation Fig 8
  • Grade I: renal pelvic dilation with or without infundibula visible Fig 9
  • Grade II: renal pelvic dilation with calices visible Fig 10, Fig 11
  • Grade III: renal pelvis and calyces dilated Fig 12
  • Grade IV: features of grade III with parenchymal thinning Fig 13
  • Ultrasound grading of hydronephrosis correlates with the severity of cortical damage or the decrease in renal function on postnatal renal scan.

Fig 8:  Schematic drawing of grade 0: no dilation

Fig 9: Schematic drawing of grade I: renal pelvic dilation without calices visible

Fig 10:  Schematic drawing of grade II: renal pelvic dilation with calices visible

Fig 11: Schematic drawing of grade II: renal pelvic dilation with calices visible

Fig 12:  Schematic drawing of grade III: renal pelvis and calices dilated

Fig 13: Schematic drawing of grade IV: renal pelvis and calices dilated as well as parenchymal thining

• Proposed criteria for the diagnosis of fetal hydronephrosis in different studies:
  • Arger (1985): AP diameter >10 mm or ratio of AP diameter of pelvis/kidney >0.5
  • Corteville (1991: AP diameter >4 mm at <33 weeks or >7 mm at >33 weeks, or ratio of AP diameter of pelvis/kidney >0.28
  • Mandell (1991): AP diameter >5 mm at 20 weeks, or >8 mm at 20-30 weeks, or >10 mm at >30 weeks
  • Adra (1995): AP diameter >8 mm from 28 weeks to term
  • Ouzounian (1996): AP diameter >5 mm from 16 weeks to term
  • Dudley (1997): AP diameter >5 mm from 16 weeks to term.
• UPJ obstruction: The ureters are not dilated and the amniotic fluid volume is usually normal.
• Vesicoureteral reflux: the hydroureter will be visualized. In severe cases, the ureter may be markedly dilated and tortuous. Care must be taken not to mistake such a ureter for a dilated bowel loop. This error can be avoided by following the dilated ureter proximally to the renal pelvis and distally to the bladder.
• Dysplastic kidney, which possibly occurs with UPJ obstruction, can be diagnosed when the parenchyma becomes abnormally echogenic or cystic.
• The contralateral kidney and related obstructions, such as ureteral dilatation, bladder distension, urethral dilatation and amniotic fluid, must be carefully evaluated.

Associations: A renal pelvis dilated to >4 mm increases the risk of trisomy 21, 18, 13.

Management: Serial sonographic monitoring is necessary to assess the degree of severity. In cases of severe obstruction, early delivery may be beneficial and the delivery should occur in a tertiary center since neonatal evaluation and treatment are necessary.

Prognosis: Good for mild forms of isolated cases, but poor in cases of dysplastic kidney especially secondary to prolonged bladder outlet obstruction. Mild fetal hydronephrosis (>4 mm) at mid-trimester appears to be associated with an excellent prognosis, probably representing physiological renal pelvic dilatation. Most cases resolve before delivery. Moderate/severe (>7 mm) fetal hydronephrosis is associated with a poorer outcome.

Recurrence risk: Sporadic in the absence of a recognizable syndrome.

Bilateral Renal Agenesis

Renal agenesis is the absence of one or both kidneys, resulting from unilateral or bilateral failure of development of the ureteric bud secondary to multicystic dysplastic kidney. Bilateral renal agenesis combined with fetal deformities secondary to oligohydramnios (pulmonary hypoplasia, abnormal facies, and abnormal limb postures) is called Potter’s syndrome.

Incidence: Unilateral renal agenesis occurs in 1 per 1000 births (male/female; 1:1) whereas bilateral agenesis is a lethal anomaly, sporadically occurring in 1-4 per 10,000 births with a male to female ratio of 2-3:1.

Sonographic findings:

• Non-visualization of kidneys and bladder.
• Oligohydramnios in the second trimester, however, amniotic fluid can be relatively normal at up to 16 weeks of gestation.
• Adrenal glands filling the renal fossae, producing the lying down adrenal sign. Fig 1, Fig 2
• Absence of both renal arteries using color-flow Doppler imaging.
• Compression effects such as dolichocephaly and small thorax. Fig 2
• Careful scanning is required before making the diagnosis because the absence of amniotic fluid degrades the sonographic image.
• Potential pitfalls in the diagnosis include
  • There is an inability to image the kidneys and bladder due to the poor image quality secondary to severe oligohydramnios; amnioinfusion may be helpful in this case. Color flow is also helpful in demonstrating renal vasculature to confirm the presence of kidney.
  • In the absence of kidney, the ipsilateral adrenal is often flattened or lying down and is mistaken for a kidney. Note that the adrenal gland has no central sinus echoes and no reniform shape in the longitudinal view.
  • Pelvic kidney: It is important to scan the fetal pelvis to be sure that the kidney is truly absent rather than in an ectopic location.
  • Crossed renal ectopia may be mistaken for unilateral renal agenesis. In this case, the united kidney is bilobed, enlarged and often with findings of obstruction.
• Usually first diagnosable in the early second trimester, at 15-18 weeks.
• In case of inconclusive findings, MRI is usually helpful.

Fig 1:  Bilateral renal agenesis  Coronal scan of the abdomen: absent kidneys with oligohydramnios and adrenal gland replacement (*) (solid circle = spine)

Fig 2:  Bilateral renal agenesis   Sagittal scan of the abdomen: absent kidneys with oligohydramnios and adrenal gland replacement (arrow)

Associations: About half of the cases may have anomalies of other organ systems; all systems can be involved.

Management: Termination of pregnancy can be offered. Vaginal delivery without electronic fetal monitoring in labor is appropriate.

Prognosis: Fatal for bilateral agenesis but excellent for unilateral agenesis.

Recurrence risk: Sporadic with rare autosomal dominant inheritance.

Large Bowel Obstruction

Large bowel obstruction is generally more difficult to diagnose than small bowel obstruction because there is considerable variability in the diameter of the normal fetal colon. The colon diameters of normal fetuses can overlap with those of fetuses with abnormally distended large bowel. During the latter part of the third trimester, the normal fetal colon can be particularly prominent and striking and could be mistaken as abnormal.

Incidence: About 1 in 2000 births.

Sonographic findings:

• Dilated colon with or without dilated small bowel. Fig1
• Dilated V- or U-shaped segment of bowel in the fetal pelvis suggestive of anorectal atresia. Fig2
• Calcified intraluminal meconium. Fig3
• The sensitivity of ultrasound for the diagnosis is rather low.
• Congenital duplication of the cecum or colon shows peristaltic cyst with haustra at the lower abdomen, which may mimic colonic obstruction.

Fig 1:  Meconium ileus  Cross-sectional scan of the abdomen: Markedly dilated transverse colon (*) with meconium contents

Fig 2:  Anorectal atresia   Oblique cross-sectional scan of the abdomen: U-shaped cystic mass located at the lower abdomen (*), separated from the bladder (arrowhead=spine)

Fig 3:  Hyperechoic colon  Cross-sectional scan of the abdomen: hyperechoic colon (solid circle) (arrowhead = spine)

Associations: Related to other malformations in most cases including VACTERL syndrome, multiple congenital anomalies, caudal regression syndrome, and abnormal chromosome (15%).

Management: A careful search for associated anomalies is indicated. In general the disorder does not alter the standard obstetric care, however, the pregnancy should be managed in a tertiary center.

Prognosis: Poor overall (due to associated anomalies and prematurity), but good in isolated cases with surgical correction.

Recurrence risk: Sporadic with rare familial cases.

Meconium Peritonitis

Meconium peritonitis is a condition resulting from a small bowel perforation, leading to an inflammatory reaction of the peritoneum and intra-abdominal calcifications.

Incidence: About 1 in 2000 births.

 Sonographic findings:

• Calcifications (bright echogenic area) in the abdomen, often with ascites.
• The calcifications can occur anywhere within the peritoneal cavity but must be distinguished from intraluminal or intrahepatic calcifications. Fig1, Fig2
• Typically the calcifications are linear in nature, sometimes rimming a hypoechoic mass or pseudocyst. Fig3
• Pseudocyst, a hypoechoic mass representing extraluminal meconium, resulting from a contained bowel perforation.
• Usually associated with ascites and polyhydramnios.
• Typically diagnosed in the third trimester.
• Persistent ascites, pseudocyst or dilated bowel loop are most sensitive for predicting postnatal surgery.

Fig 1:  Meconium peritonitis  Coronal scan of the abdomen: ascites (*) and calcification (arrow) outlining the visceral structures

Fig 2:  Meconium peritonitis  Cross-sectional scan of the abdomen: ascites (*) and calcification outlining the visceral structures

Fig 3:  Meconium peritonitis  Oblique coronal scan of the abdomen: dilated large bowel loops (*) with thickened wall and ascites (arrowhead)

Associations: Cystic fibrosis is found in 15% of cases with meconium peritonitis in neonatal series but is rarely found in prenatal series. Some cases are related to pavovirus B 19.

Management: Sonographic monitoring may be beneficial in assessing fluid volume, fetal growth and the degree of the severity. Early delivery may be appropriate if there is marked bowel dilatation. The pregnancy should be managed in a tertiary center.

Prognosis: Rather good in isolated cases, with most cysts disappearing spontaneously during pregnancy.

Recurrence risk: Rare for isolated cases, but may be as high as 25% when associated with cystic fibrosis.