Fetal abdominal wall defects occur in 1 in 2000 live births . The most common abdominal wall defects are gastroschisis and omphalocele; uncommon anomalies include ectopia cordis, limb-body wall complex, cloacal exstrophy, and urachal cyst (table 1). Widespread use of antenatal ultrasound examination and maternal serum alpha-fetoprotein screening has led to increasing detection of these defects in the second trimester of pregnancy. Moreover, with the increased use of first trimester sonography for the assessment of nuchal translucency in screening for Down syndrome, the diagnosis of these defects is shifting to as early as 11 to 14 weeks of gestation. The early gestational scan should be able to detect all cases of omphalocele, gastroschisis, and body stalk anomaly .
During the 4th to 5th week of development, the flat embryonic disk folds in four directions and/or planes: cephalic, caudal, and right and left lateral. Each fold converges at the site of the umbilicus, thus obliterating the extraembryonic coelom. The lateral folds form the lateral portions of the abdominal wall and the cephalic and caudal folds make up the epigastrium and hypogastrium [3,4]. Rapid growth of the intestines and liver also occurs at this time. During the 6th week of development (or eight weeks from the last menstrual period), the abdominal cavity temporarily becomes too small to accommodate all of its contents, resulting in protrusion of the intestines into the residual extraembryonic coelom at the base of the umbilical cord. This temporary herniation is called physiologic midgut herniation (PMH) and is sonographically evident between the 9th to 11th postmenstrual weeks (image 1). Reduction of this hernia occurs by the 12th postmenstrual week; beyond the 12th week a midgut herniation is no longer physiological .
A simple midline omphalocele develops if the extra-embryonic gut fails to return to the abdominal cavity and remains covered by the two-layer amnionic-peritoneal layer into which the umbilicus inserts [3,4,6]. In contrast to fetal bowel, the liver does not have a physiological migration outside of the abdominal cavity during development. Therefore, the liver is never present in physiologic midgut herniation. However, if the lateral folds fail to close, a large abdominal wall defect is created through which the abdominal cavity contents, including the liver, can herniate [3,4]. The result is a liver-containing omphalocele (see 'Containing liver' below)
SONOGRAPHIC APPROACH TO DIAGNOSIS
Whenever an abdominal wall defect is identified prenatally, five issues should be addressed to help categorize the defect .
●Does the defect include a limiting membrane? A membrane helps to distinguish omphalocele from gastroschisis; however, membranes occasionally rupture in-utero.