17.3 Fetal Anatomic Survey: Heart, Chest, GI, GU, Skeleton & Fetal Position

Beyond the head and spine, the standard survey sweeps the chest, abdomen, genitourinary tract, and limbs, and finishes with the obstetric summary — fetal number, cardiac activity, lie, and presentation. Each organ system has a small set of views and thresholds the ARRT exam expects you to know precisely.

Heart and Chest

The four-chamber view is a transverse plane of the chest just above the diaphragm. Normal findings:

• The heart occupies about one-third of the thoracic area, with the apex pointing to the left (levocardia) and a cardiac axis of approximately 45° ± 15° from the midline
• Four chambers of roughly equal size; the left atrium is the chamber closest to the spine/descending aorta (a situs clue)
• The foramen ovale flap opens into the left atrium
• The tricuspid valve inserts slightly more apical (closer to the apex) than the mitral valve — a key normal landmark; equal insertion suggests Ebstein anomaly or an AV canal defect
• A moderator band identifies the morphologic right ventricle near the apex
• An intact interventricular septum and two separate atrioventricular valves

Situs is confirmed by showing the stomach and cardiac apex on the same (left) side; discordance raises concern for heterotaxy. The outflow tracts are then assessed: the left ventricular outflow tract gives rise to the aorta, the right ventricular outflow tract to the pulmonary artery, and the two great arteries should cross (criss-cross) at their origins — parallel great arteries suggest transposition. The three-vessel-trachea view compares the pulmonary artery, aorta, and superior vena cava in size order. In the chest, the diaphragm appears as a smooth hypoechoic band separating thorax from abdomen; herniation of the stomach or liver into the chest with mediastinal shift indicates a congenital diaphragmatic hernia (usually left-sided). The lungs should appear homogeneous and slightly more echogenic than the liver.

Gastrointestinal Tract and Abdominal Wall

• Stomach: a fluid-filled bubble in the left upper quadrant, normally visible from about 14 weeks. A persistently absent or small stomach suggests esophageal atresia or impaired swallowing (often with polyhydramnios).
• Double bubble: two fluid-filled structures — stomach plus dilated proximal duodenum — indicate duodenal atresia. It carries a strong association with trisomy 21 (roughly one-third of cases) and presents with polyhydramnios, typically after 24 weeks.
• Echogenic bowel: bowel as bright as bone is a soft marker associated with trisomy 21, cystic fibrosis, cytomegalovirus (CMV) infection, growth restriction, and swallowed blood.
• Bowel dilatation beyond ~7 mm small bowel or progressively enlarging colon suggests obstruction; normal colon may reach ~18–20 mm near term.

The abdominal wall is examined at the cord insertion. The two classic defects are high-yield exam contrasts:

Genitourinary Tract

The kidneys are reniform retroperitoneal structures flanking the spine, documented in transverse and longitudinal planes. The renal pelvis is measured in AP diameter on a transverse view with thresholds of <4 mm in the second trimester and <7 mm in the third trimester; exceeding these defines pyelectasis, which — when bilateral or progressive — warrants follow-up for urinary tract obstruction or aneuploidy risk. Unilateral pyelectasis is far more reassuring than bilateral dilation with caliectasis. The bladder should be seen in the pelvis, filling and emptying on a cycle of roughly 30–45 minutes; color Doppler of the two umbilical arteries flanking the bladder confirms both vessel count and bladder identity. A persistently absent bladder with oligohydramnios suggests bilateral renal agenesis or severe renal dysfunction (Potter sequence), since fetal urine is the main source of amniotic fluid after mid-gestation.

Skeleton and Limbs

All four limbs are surveyed, confirming three segments each (femur/tibia-fibula/foot; humerus/radius-ulna/hand) plus the hands and feet. Club foot (talipes equinovarus) is diagnosed when the foot remains fixed in the same plane as the tibia and fibula on a true sagittal view of the lower leg — the sole faces inward and the deformity persists on repeated views (distinguishing it from transient positional flexion). Club foot may be isolated but is associated with spina bifida and trisomy 18. Radial ray defects, clenched hands with overlapping fingers (trisomy 18), polydactyly (trisomy 13), and sandal-gap toes are additional musculoskeletal soft markers. Markedly shortened, bowed, or fractured long bones suggest a skeletal dysplasia.

Fetal Number, Position, and Documentation

Every study documents fetal number (and, in multiples, chorionicity and amnionicity, dividing membranes, and comparative biometry), cardiac activity (rate and rhythm, ideally by M-mode), placental location, amniotic fluid volume, and fetal position terminology:

• Lie — the relationship of the fetal long axis to the maternal long axis: longitudinal, transverse, or oblique
• Presentation — the part entering the pelvis first: cephalic (vertex, the normal term presentation), breech (frank, complete, or footling), or shoulder (with transverse lie)
• Position — the relationship of a fetal reference point (e.g., occiput) to the maternal pelvis, e.g., occiput anterior

Presentation often changes freely before ~34–36 weeks and becomes clinically decisive near term, where it directly affects delivery planning.

Documenting Cardiac Activity, Rate and Rhythm

Cardiac activity is a required documented element of every obstetric examination, not merely an observation — and documentation means recording a rate, not just noting that the heart beats.

• Normal second- and third-trimester fetal heart rate is 110 to 160 bpm. A sustained rate below 110 bpm is bradycardia; a sustained rate above 160 bpm is tachycardia.
• Measure with M-mode, placing the cursor so it crosses an atrial wall and a ventricular wall simultaneously. That single trace lets you compare atrial and ventricular rates, which is what separates sinus tachycardia from atrial flutter with block, or benign irregularity from complete heart block. Spectral Doppler across an inflow and an outflow tract is an accepted alternative.
• M-mode is preferred over spectral Doppler for routine rate documentation because it deposits far less acoustic energy, consistent with ALARA.
• Rhythm matters as much as rate. Isolated premature atrial contractions are common and usually benign, appearing as a dropped or early beat on M-mode. A sustained tachyarrhythmia — typically supraventricular tachycardia above 200 bpm — is a leading cardiovascular cause of non-immune hydrops and warrants prompt referral. Sustained bradycardia raises concern for complete heart block, associated with maternal anti-Ro/SSA antibodies and with structural heart disease such as heterotaxy syndromes.