Fetal Colon


Normal Measurements

The fetal colon can be visualized as early as 22 weeks as a tubular structure around the perimeter of the fetal abdomen. [1] The colon demonstrates no peristalsis and the diameter is related linearly to gestational age, reaching an 18 mm maximum diameter at term. [1] The diameter of the descending colon as a function of gestational age has a sigmoid curve.  The mean values and the 95% limits have been described. [2] Overall, the fetal small bowel rarely exceeds 6 mm and the colon lumen rarely exceeds 23 mm, and haustra folds are frequently demonstrated. [3]


Screening Ultrasound

Screening of the fetal abdomen usually is not done in a systematic manner.  Ascites, dilatation of bowel, or fluid filled cysts are detected in the fetus, but esophageal atresia and anorectal malformations are frequently missed. [4] Coronal views of the esophagus, small bowel, and colon may be beneficial as well as median cuts of the rectum.

Recently two scanning techniques to assess the fetal anus have been proposed [5]:

1.  A tangential scan posterior to the external genitalia of the fetal perineum.  The anus is seen as a hypoechoic ring which represents the wall of the anal canal with a central echogenic dot representing the lumen.

2.  A coronal scan of the fetal pelvis is useful to assess the anal canal in continuity with the rectum.


Anal Atresia

In cases of imperforate anus or anorectal atresia, dilatation of the fetal colon can be seen as early at 12 weeks. [6],[7] In some cases of anal atresia, the colon may be transiently dilated as early as 16 weeks and then revert to normal colonic appearance by 20 weeks. [8]

In many cases of anorectal atresia,  associated malformations are present.  In one study, eleven of 12 fetuses (92%) with anorectal atresia had malformations related to the VACTERL syndrome (vertebral defects, anal atresia, tracheoesophageal fistula with esophageal atresia, radial and renal dysplasia, and limb malformations), and in most cases, ultrasound is able to define one or more of these malformations. [9]

Imperforate anus may be seen with intraluminal calcifications in dilated loops of intestines in the presence of a urinary tract abnormality [10], and other rare combinations of dilated large bowel loops, echogenic structures within the lumen, and oligohydramnios suggest anal-urethral atresia with vesico-rectal fistula. [11]


Colonic Atresia

The transverse colon may be dilated in cases of colonic atresia [12], and colonic atresia with obstruction and rupture may present as isolated fetal ascites. [13]



Notochord syndrome is a combination of a spinal cystic lesion with a protruding segment of bowel, and variants may include imperforate anus with recto-urethral fistula. [14]

Dilatation of the sigmoid colon and the finding of a beak-like nose is suggestive of the rare autosomal recessive disorder, Johanson-Blizzard syndrome. [15]


Hirschsprung’s disease

Prenatal diagnosis of Hirschsprung’s disease is suggested by dilated bowel loops, increased abdominal circumference, and mild polyhydramnios. [16]


Megacystis-microcolon-intestinal hypoperistalsis syndrome (MMIHS)

Megacystis-microcolon-intestinal hypoperistalsis syndrome (MMIHS) is a rare, usually lethal, autosomal recessive disorder characterized by dilatation of the urinary tract, and fetal colon with normal amniotic fluid volume. [17] The syndrome has also been reported in association with Trisomy 18. [18] Microscopic findings indicate a visceral myopathy in the bowel and bladder of infants affected with MMIHS, [19] and therefore, the obstruction of the bladder and intestines is functional. [20] Most cases occur in females. [21]

A number of cases of MMIHS have been detected in utero.  Ultrasound findings include persistent gastric dilatation, while pathologic findings include: megacystis, bilateral hydronephrosis and megaureters, short bowel, microileum, microcolon, and malrotation of the intestines. [22]

MRI and enzyme analysis help identify obstruction and define MMIHS. [23] In the ultrasound assessment, the dilatation of the fetal bladder obscures the dilatation of the colon, but the diagnosis is suggested by a normal sodium and chloride from the bladder aspirate, normal amniotic fluid volume, and findings of colon dilatation. [24] Clinical presentation of MMIHS may include a rapidly progressive bilateral hydronephrosis and bladder dilation but normal amniotic fluid volume, [25] and in some cases, polyhydramnios does not appear until the third trimester. [26]

Finally, a positive family history may be useful in cases of a cystic structure in the pelvis, enlarged stomach, dilated bowel, and prominent renal pelvis [27], and MMIHS may be present in combinations with other renal malformations such as bilateral renal duplicated collecting systems. [28]


Chloride Diarrhea

Congenital chloride diarrhea is a rare hereditary syndrome [29] characterized on ultrasound by dilated loops of bowel, fluid within the bowel, and polyhydramnios.  MRI may be particularly useful if the condition is suspected since distinction is possible between fluid within the bowel and meconium. [30] Prenatal diagnosis is warranted because of the need for identification and neonatal treatment.

Similarly to chloride diarrhea, microvillus inclusion disease also may present with dilated bowel loops and polyhydramnios.  In this condition, molecular analysis is possible and may guide genetic counseling. [31]



Duplication of the transverse colon is unusual for bowel duplication. [32] Only 13% of duplications occur in the colon, while the most common site is the ileum. [33] A significantly dilated large bowel with hyper-peristalsis suggests complete duplication of the colon, [34] and cystic duplication of the colon may be confused with an ovarian cyst which is a more common entity. [35] Unusual presentations include cystic duplication of the descending colon with sigmoid colon enteric communication [36], and combined duplication of the bladder and bowel from ileum to rectum. [37]


Cloacal dysgenesis sequence (CDS)

Cloacal dysgenesis sequence is seen in 1:50000 to 250,000 neonates and presents with a phallus-like structure, smooth perineum, and absence of urethral, vaginal, and anal openings. [38] Ultrasound findings include:  echogenic bowel, dilated colon, perineal mass, dilated bladder, and fetal hydrops. [38] Infants with cloacal dysgenesis sequence (CDS) have no egress for urine and usually have renal failure and pulmonary hypoplasia, but survival has been reported. [39]


Cloacal malformation

Cloacal malformations are characterized by a communication between the gastrointestinal, urinary, and genital structures resulting in a single opening on the perineum. [40] Ultrasound findings include massive polyhydramnios and dilatation of the bowel. [41] Cloacal atresia may also exist with multiple atresias of the small bowel and the colon, representing a possible new autosomal recessive defect. [41]



A hyper-echogenic colon [42] and a tubular echogenic mass noted at < 36 weeks and filling contiguous portions of the fetal colon have been associated with cystinuria. [43] The antenatal biochemical expression of cystinuria and its relationship to fetal hyper-echogenic colon is discussed elsewhere. [44] Finally, an echogenic colon may also reflect the presence of an entero-urinary fistula. [45]



A future treatment option may be gene delivery to the fetal gut by ultrasound guided gastric injection, which could ultimately prevent the early-onset of intestinal diseases such as cystic fibrosis. [46]




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