Embryology and Normal Ultrasonographic Appearance of the gastrointestinal
canal
The embryonic gut has three major parts: the foregut (esophagus, stomach,
proximal duodenum, liver, and pancreas), the midgut (distal duodenum, jejunum,
ileum, and proximal colon), and the hindgut (distal colon, rectum, anus, and parts of
the vagina and bladder). Dilation of the foregut during the 6th menstrual week leads
to the formation of the stomach, which then descends into the abdominal cavity by
the 8th to 9th week. The midgut typically returns to the abdominal cavity by the 11th
week. Although intestinal peristalsis begins by week 11, it can rarely be visualized
ultrasonographically before the second trimester.
Fetal swallowing is evident before the end of the first trimester, and beyond the
14th week, the stomach should be visualized in nearly all normal fetuses. With a
transvaginal probe, the stomach can be visualized as early as the 9th week of
gestation. In contrast, the normal fetal esophagus is not generally visualized
ultrasonographically.
The small bowel and large bowel can be imaged and distinguished from each other,
particularly in the third trimester. The more centrally located small bowel is often
difficult to discern in normal fetuses, appearing as a relatively homogeneous, slightly
echogenic mass. The small bowel remains more echogenic than the large bowel until
near term, displaying active peristalsis during the third trimester. Late in the third
trimester, small bowel loops are commonly identified, generally measuring less than
15 mm in length and rarely more than 5 mm in diameter. Improvements in ultrasound
imaging technology have permitted the imaging of small bowel loops as early as 14
week's gestation.
The colon frames the fetal abdomen, appearing as a continuous tube with a
hypoechoic lumen. Meconium creates this hypoechoic appearance, in contrast to the
echogenic bowel wall. In fact, normal colon with liquid meconium is often mistaken for
pathology (eg, dilated bowel, cysts). The colon exhibits far less peristalsis than the
small intestine. By the middle of the third trimester, colonic haustra can be identified
in nearly all fetuses, despite the fact that clinically, haustra are generally poorly
developed in newborns. The diameter of the large bowel increases in linear fashion
from 3 to 5 mm at 20 week's gestation to up to 20 mm at term. Although colonic
grading has been attempted based on comparative echogenicity, the clinical utility of
this approach has not been demonstrated.
The liver is the dominant organ in the upper fetal abdomen, the area for standard
abdominal circumference measuremen. The gallbladder is often overlooked or
misrepresented ultrasonographically (under the assumption that it represents an
intrahepatic vein). Although it is a sonolucent structure, its ovoid or conical shape and
lack of flow on color Doppler or power angiography, as well as its location inferior and
to the right of the intrahepatic segment of the umbilical vein, should help distinguish it
The spleen is another upper abdominal organ that is often overlooked or mistaken for
an abnormal solid left-sided mass. Although not always easily visualized, it can
usually be seen throughout the second half of gestation, posterior to the stomach and
lateral and superior to the left kidney. It is homogeneous in appearance and slightly
less echogenic than the liver, similar to the kidney. The pancreas is sometimes
visualized in the third trimester as well, especially with the fetus in a supine position.
ABSENT STOMACH
Inability to visualize the stomach in the second and third trimesters of pregnancy
requires further examination. Repeated examination over time (several hours or days
later) confirms a normal fetal stomach in about half of cases. Esophageal atresia
should always be considered when polyhydramnios is accompanied by failure to
visualize the stomach bubble. The differential diagnosis for this problem is
summarized in the table.
Differential Diagnosis of Absent Stomach Bubble
Empty stomach (time)
Esophageal atresia with or without tracheoesophageal fistula
Diagphragmatic hernia
Situs inversus
Oligohydramnios
Facial clefts
Central nervous system disorders
ESOPHAGEAL ATRESIA
Esophageal atresia should always be considered when polyhydramnios is
accompanied by failure to visualize the stomach bubble. However, 90% of cases are
associated with a communicating tracheoesophageal fistula, usually with a visible
stomach. The gastric mucosa alone may produce enough fluid to allow visualization
of the stomach in some cases of atresia without fistula. The blind pouch of the
proximal esophageal segment has been seen infrequently, as has regurgitation after
fetal swallowing during real-time observations. Associated anomalies are seen in
nearly 60% of cases, with other gastrointestinal (28%), cardiac (24%), and
chromosomal (19%) anomalies being the most common. As with most intestinal
atresias, esophageal atresia is rarely diagnosed before the third trimester. Although
the survival rate is particularly poor overall (less than 50%), the prognosis is much
improved with isolated esophageal atresia (85% survival rate).
DUODENAL ATRESIA
Duodenal atresia, with an incidence of about 1 in 5000 births, is the most common
intestinal obstruction encountered in the perinatal period. The lesion is most
commonly caused by one or more membranes interrupting the duodenal lumen. The
classic double-bubble appearance is almost invariably associated with
polyhydramnios, makes the ultrasonographic diagnosis relatively straightforward. It is
extremely important, however, to attempt to demonstrate the communication between
the stomach and the first part of the duodenum.
Associated anomalies are the rule rather than the exception with this disorder,
occurring in more than half of cases. In particular, 30% of fetuses with duodenal
atresia have Down syndrome; hence, prenatal diagnosis should be offered. By way of
contrast, however, only 10% of Down syndrome fetuses have duodenal atresia.
Other anomalies commonly associated with Down syndrome include skeletal,
cardiac, and other gastrointestinal malformations. Renal anomalies are seen in about
8% of cases, and growth restriction is common.
SMALL BOWEL OBSTRUCTION
The incidence of small bowel obstruction is about 1 in 5000 births, but the
prevalence of in utero or ultrasonographic detection may be somewhat higher. It is
often difficult, if not impossible, to differentiate jejunal from ileal atresia, although the
extent of bowel dilation is often a clue (more loops are present with ileal
abnormalities. Polyhydramnios is often the initial clue to a small bowel obstruction
and is present in most cases. Unfortunately, as with duodenal atresia, jejunal and
ileal obstructive problems are invariably diagnosed late in pregnancy, specifically in
the third trimester. Unlike duodenal atresia, however, nongastrointestinal anomalies
are rare in these cases. Other intestinal tract anomalies are common, with up to 45%
exhibiting malrotations or enteric duplications, for example. Distal ileal obstructions
appear to have the most favorable prognosis.
Multiple dilated loops of small bowel (more than 7 mm in diameter) are the typical
features in these cases, often with increased peristalsis. Differentiating small bowel
from large bowel obstruction is usually possible by the location of the loops, the
absence of haustra in small bowel, and the presence of polyhydramnios. Colonic
obstructions, however, are uncommonly diagnosed in utero, and Hirschsprung's
disease (congenital aganglionic megacolon) is likely to present with dilated small
bowel and polyhydramnios, making it difficult to differentiate from a jejunoileal lesion.
ANORECTAL ATRESIA
The incidence of anorectal malformations is about 1 in 5000 live births. The
prenatal diagnosis of imperforate anus and other anorectal anomalies has been
reported about 15 times. Generally, the diagnosis has been as part of a syndrome of
multiple anomalies (eg, caudal regression syndrome, VACTERL or VATER
syndromes, McKusick-Kaufman syndrome). VACTERL syndrome may have a
number of concurrent abnormalities, including vertebral abnormalities, anorectal
atresia, cardiac anomalies, tracheoesophageal fistulas, esophageal atresia, renal
anomalies, and limb abnormalities.
MECONIUM ILEUS
Meconium ileus results from a distal ileal obstruction with abnormally thick, viscous
meconium. When this is present, the fetus is almost certain to have cystic fibrosis
(CF), although only 10% to 15% of infants with CF have the condition. The typical
appearance of dilated ileum, with a normal jejunum, collapsed colon, and
polyhydramnios, may be indistinguishable from other small bowel obstructions. In
some cases, however, the bowel presents as an echogenic mass.
MECONIUM PERITONITIS
Meconium peritonitis is a condition resulting from a small bowel perforation. The
perforation generally leads to a chemical peritonitis, which results in intraabdominal
calcifications that are the characteristic ultrasonographic feature in this condition.
ABDOMINAL CYST: DIFFERENTIAL DIAGNOSIS
When faced with the dilemma of an anechoic, presumably cystic mass in the fetal
abdomen, a systematic approach is critical to making an appropriate diagnosis.
Preliminary assessments to make include:
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Location and orientation of the mass
Relation to other abdominal organs
Size and shape of the lesion
Fetal gender
Wall and contents of the cyst
In addition, the change in appearance over time may be extremely helpful in difficult
cases. (Some of these cystic-appearing abnormalities are covered in the preceding
sections on bowel obstruction or meconium pseudocyst.)
Ovarian cyst is among the more common anechoic abdominal anomalies seen in a
female fetus in the third trimester. They are usually, although not exclusively, located
within the lower fetal abdomen. They may change in position in the abdomen or
pelvis and may undergo torsion because they are often on a pedicle. It is thought that
these cysts arise as a result of hormonal stimulation from the mother and placenta.
Mesenteric or omental cysts are usually located in the mid-abdomen and are
frequently mobile. Enteric duplication cysts are generally tubular in appearance, do
not communicate with the bowel, and are more common in male fetuses.
Sacrococcygeal teratomas are solid masses, but they may have cystic components.
They originally arise from the deep pelvis coccyx and may have both intrapelvic and
extrapelvic components.
Ascites
HEPATOSPLENOMEGALY
Enlargement of the liver or spleen may occur secondary to an isolated finding such
as a liver tumor (hemangioendothelioma) or systemic abnormalities such as fetal
hydrops.