Normal Sonographic Anatomy

In each trimester of the pregnancy, specific landmarks of normal fetal anatomy should be verified by obstetric ultrasound (US) [1, 2].

During the First Trimester

The stomach appears as a cystic structure in the left hypochondrium. It should be constantly visible at around 11-12 weeks.

The small bowel herniates physiologically through the umbilical cord at around 8-9 weeks. The herniation should have resolved by 11-12 weeks.

The rest of the digestive tract is not visible at this stage.

The bladder is the other cystic structure that should be visible during the first trimester. It is located medially in the fetal pelvis and is visible around 8-9 weeks.

The kidneys are visible around the end of the first trimester and appear as hyperechoic bean-shaped structures in each perilumbar area.

At this stage, the amniotic fluid is produced by the placenta and does not reflect the renal function.

During the Second Trimester

The stomach is globally unchanged. The small bowel gets progressively filled with meconium. It will be visible as small parallel tubular echogenic structures, mainly in the left flank. The colon is not visible until the end of the second trimester.

The bladder should always be visible during the mid trimester examination. It should show normal cycles of filling and emptying (during a single examination).

The kidneys now appear more obvious, since their global echogenicity decreases and since cortico-medullary differentiation begins (and should be constantly present by 17-18 weeks).

The rest of the abdomen is 'filled' with the liver and the spleen, both appearing as homogenous triangular structures. The adrenals are triangular hypoechoic masses located on the top of the kidney. The pancreas and the abdominal vessels complete the evaluation of the fetal abdomen.

During the Third Trimester

The esophagus is visible when distended by amniotic fluid.

The stomach shows obvious peristaltic waves. It may be filled with echogenic material (gastric pseudomass) corresponding to swallowed debris.

The small bowel is now filled with swallowed amniot-ic fluid and appears as small cystic structures of variable sizes due to normal peristalsis.

The colon is progressively visible as it fills with hypoechoic meconium and by 27-28 weeks it can be followed from the cecum to the recto-sigmoid. Its diameter widens progressively (it should be less than 2 cm).

The appearance of the bladder and kidneys should be stable. The kidneys grow progressively with gestational age. Their length measures around 3 cm at 30 weeks.

The adrenals are large and display a corticomedullary differentiation [1, 2].

The liver occupies a large part of the fetal upper abdomen. The gallbladder is almost always visible.

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