The Fetal Anterior Abdominal Wall

Embryology of the Abdominal Wall

By the end of the fifth week of development, an embryo is a flat disk consisting of three layers: ectoderm, mesoderm, and endoderm. In the sixth week, a process called folding helps the embryo to transform itself into a cylindrical shape ( Fig. 62.1 ). This transformation is a critical part of the process of closing the abdominal wall.

As the embryo folds at the cranial end, the base of the yolk sac is partially incorporated as the foregut, which later develops as the pharynx, lower respiratory system, esophagus, stomach, duodenum (proximal to the opening of the bile duct), liver, pancreas, gallbladder, and biliary duct system ( Fig. 62.2 ).

The growth of the neural tube causes the embryo to fold at the caudal end, incorporating part of the yolk sac as the hindgut, which turns into the cloaca. It also causes the connecting stalk (located at the tail) to move to the ventral surface of the embryo, incorporating the allantois into the umbilical cord. The derivatives of the hindgut are the distal part of the transverse colon, the descending colon, the sigmoid colon, the rectum, the superior portion of the anal canal, the epithelium of the urinary bladder, and most of the urethra ( Fig. 62.3 ).

The sides of the embryo fold, leading to the formation of the lateral and anterior abdominal wall. The midgut is the primordium of the small intestines (including most of the duodenum), cecum, vermiform appendix, ascending colon, and the right half to two-thirds of the transverse colon. The connection of the yolk sac and body stalk will form the umbilical cord at the ventral region of the embryo. Expansion of the amniotic cavity will cover the umbilical cord by the amnion. Fusion of the midline begins during the seventh week of development and is completed by the eighth week.

The umbilical veins drain the placenta, body stalk, and the evolving abdominal wall. During the seventh week, the hepatic bud enlarges, and the right umbilical vein atrophies. The proximal portion of the left umbilical vein between the subhepatic portion and the common cardinal vein also atrophies. Branches of the aorta now replace the nutritive function of the umbilical veins with respect to the developing abdominal wall. The superior mesenteric artery is formed from the right omphalomesenteric artery.

Umbilication hernia of the bowel occurs during the eighth week of development as the midgut extends to the extraembryonic coelom in the proximal portion of the umbilical cord. The midgut grows faster than the abdominal cavity at this stage because of the increased size of the liver and kidneys. Thus the herniation develops. The intestines return to the abdominal cavity by the 12th week of gestation.

Sonographic Evaluation of the Fetal Abdominal Wall

It should be possible to detect abdominal wall defects in utero because the defects form early in embryologic development. It is very important to image the cord insertion site and the fetal anterior abdominal wall to evaluate for the presence or absence of such defects. If the urinary bladder and pelvis are evaluated closely, the diagnosis of bladder and cloacal exstrophy may also be made with sonography. The most common types of abdominal wall defects are gastroschisis, omphalocele, and umbilical hernia. Other types of abdominal ventral wall defects include ectopia cordis, pentalogy of Cantrell, bladder and cloacal exstrophy, amniotic band syndrome, and the limb–body wall complex. The following questions should be routinely answered:

• 1.

  Is a limiting membrane present?

• 2.

  What is the relation of the umbilical cord to the defect?

• 3.

  Which organs are eviscerated?

• 4.

  Is the bowel normal in appearance?

• 5.

  Are other fetal malformations evident?

Abnormalities of the Anterior Abdominal Wall

Abdominal wall defects cause distortion of the normal contour of the ventral or anterior surface of the fetal abdomen. Table 62.1 summarizes the typical sonographic features and associated conditions of fetal abdominal wall defects.

The most common abdominal wall defects, omphalocele , umbilical hernia (a form of omphalocele), and gastroschisis , will be presented. The incidence of omphaloceles is approximately 1 in 4000 live births. Overall, gastroschisis has an incidence of 12 per 10,000, although only rarely are affected infants born to older mothers. The role of the perinatal team is to distinguish among these lesions because clinical management, associated anomalies, delivery, and postnatal surgical survival vary, depending on the specific type of abdominal wall defect. Box 62.1 outlines what the sonographer should investigate to distinguish between omphalocele and gastroschisis.