Normal small bowel
The normal fetal bowel varies in appearance during gestation. The small bowel appears brighter than the liver. The fetal small bowel can be seen sonographically as early as 12 weeks of gestation. Between 12-16 weeks, it appears homogenous.
Meconium accumulates in the small bowel from the early 2nd trimester.
As pregnancy progresses, the bowel becomes more heterogenous, centrally located and well defined with echogenic walls and hypoechoic contents. Late in pregnancy discrete fluid filled loops of bowel can be seen in virtually all fetuses.
Individual segments of small bowel should not exceed approximately 7-10 mm in diameter and 15 mm in length.
Peristalsis within the fetal bowel can be seen as early as 16 weeks of gestation.
Normal large bowel
The large bowel is not clearly recognizable with prenatal sonography until the early third trimester. Thereafter it becomes increasingly easy to recognize.
The colon frames the fetal ab domen, appearing as a continuous tube with a hypoechoic lumen.
The normal colon can be erroneously considered dilated small bowel. The identification of haustral folds, peripheral location, hypoechoic appearance and minimal peristalsis should allow small bowel to be distinguished from colon. The large bowel is seen ascending on the right side in the fetal abdomen to the hepatic flexure, transverse to the splenic flexure and inferior to the sigmoid and rectum.
The large bowel exhibits wide variation in diameter but rarely exceeds 23-26 mm.
The dilated bowel
The fetal bowel can be altered by a number of pathological processes. The commonest pathology is a bowel dilatation characteristically recognized as one or more tubular structures within the fetal abdomen. Bowel dilatation is due to obstruction which may either
Mechanical obstruction – result from either
· Intrinsic lesions – like absent (atresia) or partial recanalization (stenosis) of the intestine. In cases of atresia, the two segments of the gut may be either completely separated or connected by a fibrous cord. In cases of stenosis, the lumen of the gut is narrowed or the two intestinal segments are separated by a septum with a central diaphragm.
· Extrinsic obstructions – are caused by malrotation of the colon with volvulus, meconium ileus, and agangliosis.
Functional causes - for bowel obstruction include congenital chloride diarrhoea, microcystis megacolon intestinal hypoperistalsis syndrome.
· Most intestinal obstructive disorders do not occur until the third trimester (a second trimester scan is usually normal).
· Multiple interconnecting over distended bowel loops.
- Individual small bowel loops of >15 mm in length or 7 mm in diameter and large bowel > 23 mm in diameter
- The number of dilated bowel loops depends on the level of obstruction (the lower the level the greater the number).
- Peristalsis in the obstructed loops confirms that we are indeed looking at bowel.
· Polyhydramnios (more common in higher obstructions).
· Complications include bowel obstruction, perforation and ischemic necrosis due to vascular compromise which may be seen as intra abdominal calcification, ascites and matted bowel loops.
The following conditions can mimic a dilated bowel
1. Normal meconium filled colon mistaken for abnormally dilated small bowel
2. Normal dilatation of intestinal loops due to peristalsis
3. Renal tract anomalies – multicystic dysplastic kidney, dilated ureter
Obliteration of single or multiple segments of small or large bowel causing obstruction and segmental dilatation.
— 1 per 2000 live births
— More than one third of congenital bowel obstructions result from intestinal atresias
— In about half the cases there is small bowel obstruction and in the other half, anorectal atresia.
Small bowel obstruction
a. Jejuonoileal atresia: The most frequent site of small bowel obstruction is distal ileum (35%), followed by proximal jejunum (30%), distal jejunum (20%), and proximal ileum (15%).
Jejunoileal atresia is commonly categorized as follows
- Type I – Membrane
- Type II – Blind ends joined by fibrous cord
- Type IIIa – Disconnected blind end
- Type IIIb – Apple-peel deformity
- Type IV – Multiple, string of sausages
b. Meconium ileus
— Impaction of abnormally thick and sticky meconium in the distal ileum-functional obstruction
— Nearly all newborns have cystic fibrosis
— Dilated bowel segments containing echogenic meconium - usually seen after 26 weeks
— USG findings include
- Dilated bowel loops
- Meconium peritonitis
- Abnormal areas of increased echogenicity
— All the above findings are non specific and definitive diagnosis is difficult.
Large bowel obstruction
Large bowel obstruction is rare and more difficult to diagnose because of wide variations in the diameter of a healthy fetus’s colon. Abnormalities of the large bowel causing a bowel dilatation include Anorectalatresia and Hirschsprung disease.
a. Anorectal atresia
· 1 in 2000 births
· Rarely seen as dilated colon in the lower abdomen or pelvis
· Calcified intraluminal meconium pellets seen either proximal or distal to the site of obstruction
· Amniotic fluid volume usually normal. May be reduced when associated with bilateral renal disorders.
b. Hirschsprung disease
· Characterized by congenital absence of intramural myenteric nerve ganglia and sympathetic nerve plexus in a bowel segment.
· 1 in 10000 to 20000.
· Rarely presents prenatally, but when it does it is usually due to total colonic agangliosis.
· Sonographic features are non specific and rare. May be seen as dilated bowel segments and mild polyhydramnios
· Bowel loops twist around the mesenteric artery or it’s branches.
· The ischemic necrosis may lead to intestinal atresia
· Characteristic ultrasound marker- ‘Whirlpool sign’ – the twisted bowel loop and it’s accompanying mesentery and mesenteric vessels exhibit a whirlpool like configuration. But this sign is not depicted in all cases.
· Other non specific sonographic appearance includes – dilated bowel loops, ascites, polyhydramnios, intra abdominal cystic mass.
Congenital chloride diarrhoea
· Autosomal recessive
· Disorder of chloride transport in distal ileum and colon
· USG – moderately dilated intestinal fluid filled loops (entire jejunum and ileum) with good peristalsis.
Ultrasound can differentiate a dilated small bowel from dilated large bowel. However the prenatal ultrasound demonstrates true insufficiencies in fixing the precise etiology.
Normal fetal bowel has a wide range of appearances. It is therefore important to be aware of the various patterns attributable to the normal gastrointestinal tract and consider the possibility of normal bowel or a process arising elsewhere in the fetus before diagnosing a gastrointestinal abnormality.
Nevertheless, a simple dilatation of the bowel may refer to a more severe disorder e.g. bowel volvulus, multiple atresia, meconium ileus or a megacystis micro colon intestinal hypoperistalsis syndrome. Hence a pre natal estimation of the problem helps us to be better prepared on arrival of the neonate.