Fetal Abdomen and Abdominal Wall Anatomy

• Fetal Abdomen and Abdominal Wall

Normal Anatomy

• The diaphragm defines the superior aspect of the abdominal cavity.
• A hypoechoic curved line separates the echogenic lungs from the liver and stomach.
• The liver is large and occupies most of the upper abdomen.
• The left lobe of the liver is larger than the right.
• The gallbladder is visible in the anterior right abdomen, inferior to the liver's margin.
• The spleen is in the left upper abdomen, posterior to the stomach, and is echogenic and homogeneous.

Abdominal Wall

• During fetal development, the viscera normally herniate into the umbilical cord base during the first trimester.
• The contents return to the abdominal cavity after mid-gut rotation.
• This process is generally complete by the 12th week.
• It is advised that anterior abdominal wall evaluations should be performed after the 12th week after the contents have returned.

Abdominal Wall Defects - Gastroschisis

• Gastroschisis involves a small defect affecting all three layers of the abdominal wall, allowing intestinal protrusion into the amniotic cavity.
• The defect is usually lateral to the umbilical cord insertion, typically on the right side.
• There is no membranous sac covering the defect.
• Gastroschisis results from a muscular defect, potentially related to vascular disruption.
• The occurrence is sporadic, with no associated anomalies or increased risk of chromosomal abnormalities.
• Variable amounts of bowel are found floating in the amniotic fluid upon sonographic evaluation.
• The umbilical cord is observed adjacent to the defect.
• There is no membranous sac covering the herniation.

Abdominal Wall Defects - Omphalocele

• This condition arises when the intestines fail to return into the abdomen during the second stage of intestinal rotation.
• The defect may contain a single loop of bowel or most of the abdominal contents.
• A membrane made up of a layer of amnion and peritoneum covers the defect.
• It is typically observed at the location of the umbilical cord insertion.
• Rupture of the sac during a vaginal delivery could cause sepsis
• Genetic counseling is recommended due to the high association between omphalocele and other chromosomal anomalies.
• Sonographic findings include an extra-abdominal mass consisting of bowel loops, liver, or other abdominal organs.
• The mass is contiguous with the umbilical cord, and a membrane covers the herniated mass
• Omphaloceles containing only bowel are more likely to be associated with chromosomal abnormalities

Bladder Exstrophy

• It involves a midline defect in the lower abdominal wall and the front wall of the bladder.
• It is generally an isolated defect.
• It causes exposure and protrusion of the urinary bladder.
• It can be associated with genital anomalies like cleft clitoris, epispadias, and wide separation of pubic bones.
• The bladder cannot be identified after over 30 minutes of scan time, despite normal AFI (amniotic fluid index)
• Possible soft tissue protrusion may be visible from the lower abdominal wall.
• Separation of the pubic bones may be observed on an ultrasound.
• Microphallus may be observed in male fetuses.

GI System - Normal Anatomy

• Esophagus: difficult to image unless the fetus is swallowing or stenosis is present.
• Stomach: Transverse views show an ovoid fluid collection in the left upper abdomen; coronal imaging detects the fundus, body, and pylorus.
• Very thin muscular layer in a fetus.
• Echoes may arise from cellular debris in the swallowed fluid within the stomach.
• Specific intestinal segments are difficult to see unless fluid offers contrast.
• Intestines: Normally mixed echogenicity appearing cystic.
• Peristalsis is seen in the late second trimester.
• Colon: Most obvious in the late third trimester.
• Meconium (bile and swallowed vernix, hair, and cells mixture) fills the colon and becomes highly echogenic.
• In the fetal abdomen, the colon is peripheral, and the small bowel is centrally located.

Esophageal Atresia

• Esophageal Atresia occurs in 1:2000 to 3000 live births and involves discontinuity of the esophagus.
• 90% are accompanied by a distal tracheo-esophageal fistula (TE fistula).
• 30–70% are associated with other congenital malformations like:
  • Cardiovascular
  • Gastrointestinal (GI)
  • Genitourinary
  • MSK anomalies
• Sonographic findings include:
  • Small or absent fetal stomach
  • Failure to demonstrate the stomach on serial sonograms
  • Polyhydramnios

Duodenal Atresia

• Duodenal obstruction is the most common perinatal intestinal obstruction.
• 65% have either a karyotypic or associated abnormality at birth, most commonly cardiac and vertebral abnormalities.
• 30% of infants with duodenal atresia have trisomy 21.
• It is more common in boys.
• A DOUBLE BUBBLE SIGN seen on sonographic findings.
• This indicates a dilated stomach and proximal duodenum, but may not be seen until after 24 weeks gestation.

Intestinal Atresia

• Obstruction of the intestine with subsequent distension of bowel loops.
• Sonographic findings consist of multiple fluid filled distended bowel loops.
• May occur anywhere along the intestinal tract all the way to the anus.
• Smal bowel internal diameter is greater than 7mm.
• There is increased peristalsis.
• Ascites and abdominal calcifications indicate perforation.
• It is typically seen in conjunction with polyhydramnios

Meconium Peritonitis

• Chemical peritonitis is caused by small bowel perforation in utero secondary to atresia or volvulus.
• Volvulus occurs when a loop of the intestine twists around itself and the mesentery that supports it, resulting in a bowel obstruction, or from a meconium ileus caused by thickened meconium.
• In 35-40% of cases, cystic fibrosis is the etiology because the meconium is sticky and thick.
• Calcifications in the fetal abdomen can be seen upon sonographic examination
• Fetal ascites is also seen.
• Meconium, the earliest stool of a mammalian infant, comprises intestinal epithelial cells, lanugo (hair), mucus, amniotic fluid, bile, and water, unlike later feces.

Midgut Volvulus

• A volvulus is an obstruction caused by the bowel twisting upon its blood supply.
• The small bowel may twist about the superior mesenteric artery's axis if it fails to return completely to the abdominal cavity and rotate correctly, leading to insufficient vascular flow past the point of blockage.
• This may also occur if the mesenteric attachments affixing the bowel to the posterior abdominal wall do not develop.
• Infarction results if this is not surgically corrected.
• Midgut volvulus is usually diagnosed in the first days of life; the infant may present with distention or obstruction, but, most typically, presents with bilious vomiting. Sonographic Imaging:
  • A fluid-filled proximal duodenum has an arrowhead twist where the descending or transverse duodenal obstruction occurs.
  • Mild polyhydramnios may be seen.
  • Echogenic mass detectable under the fetal liver.
  • Slightly dilated bowel loops are other findings.
  • A "whirlpool sign" can be seen on color Doppler.
  • The vessels are twisted around the mesenteric vessels

Hyperechoic Bowel

• The bowel is identified by its increased echogenicity on a second-trimester ultrasound (US).
• Hyperechogenicity can be diffuse or focal.
• It is similar to or sometimes greater in echogenicity than bone; there is no shadowing.
• Primarily in the lower fetal abdomen and pelvis.
• When using higher frequencies or harmonics, the bowel tends to look more echogenic.
• Use a lower frequency and avoid harmonics when checking for suspected issues. Common etiologies include:
  • Normal variant
  • Trisomy 21
  • CMV infection (cytomegalovirus)
  • Cystic fibrosis
  • Swallowed intra-amniotic blood

Large Bowel Obstruction

• The normal colon images in the third trimester and frequently without apparent intestinal peristalsis.
• Imperforate anus, meconium ileus, and Hirschsprung disease are the primary causes of large bowel obstruction.
• Hirschsprung disease is a functional disorder of the distal colon.
• The colon becomes perpetually contracted as a result.
• The colon cannot relax because the neuroenteric ganglion cells are congenitally absent in the mucosal layer, which control the relaxation phase of peristalsis, resulting in a functional obstruction.
• It is more common in males and typically affects the distal portion of the bowel.
• Other conditions that can cause the formation of a meconium plug includes maternal preeclampsia, maternal diabetes mellitus, maternal magnesium sulfate, prematurity, sepsis, and hypothyroidism.

Persistent Right Umbilical Vein

• The umbilical vein courses to the left side of the fetal abdomen.
• The umbilical vein enters the right portal vein of the liver rather than the left.
• Variation may not necessarily be pathological.
• The umbilical vein courses to the left side of the fetal abdomen in this case, as the diagnosis is only made through its right-sided connection to the portal vein, which is on the right.
• Can be seen with other anomalous conditions. -You should search for other anomalies if observed.
• Findings observed in a transverse section of the abdomen make the findings of the umbilical vein.
  1. The portal vein curves towards the liver.
  2. The fetal gallbladder is located medially to the umbilical vein ie between the umbilical vein and the stomach.
  3. The umbilical vein abnormally connects to the right portal vein instead of the left portal vein. Color doppler is necessary to identify the type of persistent righ