The echogenicity of the fetal bowel is assessed during second trimester obstetrical ultrasound examination because increased echogenicity is a marker for several fetal disorders. However, most cases have no underlying abnormality and have normal outcomes.
DEFINITION AND DIAGNOSIS
Fetal echogenic bowel refers to increased echogenicity or brightness of the fetal bowel noted on second trimester sonographic examination (image 1A-B). Hyperechogenicity can be diffuse or focal, is uniform over a well-defined area that does not shadow, and is located primarily in the lower fetal abdomen and pelvis.
Various criteria for diagnosis of bowel echogenicity have been suggested. The simplest criteria, and the one that we use, is echogenicity similar to or greater than that of adjacent bone with the ultrasound gain set to the lowest point at which bone appears white . The iliac wing is the usual standard for comparison. Some authors have defined grades of echogenicity, with the most severe form (grade 3) being as bright as bone, while grades 1 and 2 are mildly or moderately echogenic .
Transducer frequency is important when considering the diagnosis of echogenic bowel. Higher frequency transducers can make the differentiation between normal and abnormal bowel echogenicity difficult, leading to over-diagnosis of the latter. As an example, a study using both 8 and 5 MHz transducers sequentially on the same fetuses found the frequency of echogenic bowel was 31 and 3 percent, respectively . For this reason, we only diagnose echogenic bowel when using a transducer frequency of 5 MHz or below. As discussed above, echogenicity similar to or greater than adjacent bone is a subjective determination, therefore prone to interobserver and intraobserver variability .
Differential diagnosis — Echogenic bowel should be distinguished from extra-intestinal echogenic masses and calcification in the fetal abdomen. The differential diagnosis includes gastric pseudo-mass, meconium peritonitis, and meconium "pseudocyst" (image 2A-B). A description of these and other less common echogenic lesions in the abdomen can be found separately. (See "Overview of echogenic masses and calcification in the fetal abdomen".)