**2.5 Gastrointestinal tract**

The fetal abdomen differs from the abdomen of the older child or adult. The fetal abdomen is large in relation to its body dimension compared with the adult. The liver is larger with the left lobe is bigger than the right owing to its greater supply of oxygenated blood. The umbilical vein is an important US landmark. Half the blood it carries goes directly to the inferior vena cava via the ductus venosus. The remainder perfuses the liver via the left portal vein. The gall bladder is visualized as an ovoid cystic structure below the intrahepatic portion of the umbilical vein. The spleen may be demonstrated in a transverse plane posterior and to the left of the stomach. The adrenal glands are up to 20 times larger in relative size because of the presence of a fetal zone. The pelvis is small with the pelvic organs extending into the lower abdomen. Swallowing commences at 11 to 12 weeks gestational age (GA). The fetal stomach is visible from 9 weeks of gestation as a sonolucent cystic structure in the upper left quadrant of the abdomen. It should be filled with swallowed fluid by 18 weeks GA. The small bowel is moderately echogenic and centrally located. Peristalsis in small intestine loops is usually demonstrable by the third trimester. The visualized small gut usually measures just below 6 mm in width and below 15 mm in length. The large bowel is seen after 20 weeks of intrauterine life as a tubular organ in the periphery of the abdominal cavity. It gradually fills up with meconium but does not usually surpass 23 mm in caliber.

The abdominal circumference should be measured in a scan of the abdomen demonstrating the stomach and the portal sinus of the liver. The visceral situs should also be evaluated.

This is done by demonstrating the relative location of the stomach, hepatic vessels, abdominal aorta and inferior vena cava (**Figure 4**).

#### **Figure 4.**

*Obstetric ultrasound showing the fetal abdomen. Fetal abdomen: The stomach (S) and intrahepatic portion of the umbilical vein (V) are demonstrated. The spine (SP) is seen posteriorly.*

A case series of gastrointestinal abnormalities in fetuses with echogenic bowel detected during the antenatal period revealed that prenatal diagnosis of bowel abnormalities is challenging owing to the varying appearance of the bowel throughout pregnancy [26].

A related study showed that the prenatal ultrasound scan is unreliable in the detection or exclusion of fetal gastrointestinal malformations (GIM). Therefore clinicians involved in prenatal sonography or counseling should exercise caution in making such diagnoses. In this study, there were 220 confirmed cases of GIM, of which only 35 (16%) had been correctly identified prenatally. However, prenatal ultrasound was quite reliable in the detection of duodenal obstruction with 55% confirmed cases identified prenatally [27].

#### **2.6 Kidneys and urinary tract**

Detection of congenital urinary system anomalies is an important aspect of the prenatal ultrasound examination. Prenatal diagnosis of urinary tract abnormalities known to precipitate neonatal urosepsis and sequel such as renal scarring has made it possible to commence early intervention. A complete workup of the infants can be initiated early and before life-threatening complications occur.

The kidneys are visualized on sonography from as early as nine weeks of gestation and in all cases from twelve weeks. Echogenicity is high at nine weeks but reduces with advancing gestational age.

In a longitudinal scan, the kidneys are seen as elliptical structures while on axial sonograms, they are seen as rounded structures on either side of the spine. At 20 weeks, they show a hyperechoic capsule and the cortical area is slightly more echogenic than the medulla. Fat tissue normally accumulates around the kidneys as gestation progresses which enhances the borders of the kidneys in contrast with other organs. Normal ureters are rarely visualized in the absence of distal obstruction or reflux. The fetal bladder can be seen from the first trimester in more than 90% of subjects by 13 weeks (**Figure 5**).

A retrospective review of 56 children with urinary tract abnormalities detected by prenatal ultrasound revealed that more than half of the abnormalities were isolated hydronephrosis or multicystic dysplasia of the kidney [28].

#### **Figure 5.**

*Obstetric ultrasound showing fetal kidneys. Fetal abdomen at the level of the kidneys: Both kidneys (K) are seen on either side of the spine in this transverse sonogram.*

The most frequent causes of hydronephrosis in the antenatal period are ureteropelvic junction (UPJ) obstruction, ectopic ureterocele, and posterior urethral valves (PUV). Renal pelvis of more than 10 mm in anteroposterior diameter or more than half of the anteroposterior diameter of the kidney in transverse section are conclusive evidence of significant hydronephrosis.
