18.2 Major Structural Anomalies by System
Key Takeaways
- Anencephaly shows absence of the calvarium and cerebral hemispheres above the orbits (frog-eye appearance) with elevated maternal serum alpha-fetoprotein and polyhydramnios
- Spina bifida is flagged by the intracranial lemon and banana signs — frontal bone scalloping and an anteriorly curved cerebellum with an obliterated cisterna magna — before the spinal defect itself is seen
- Ventricular septal defect is the most common congenital heart defect, and tetralogy of Fallot combines a VSD, overriding aorta, pulmonary stenosis, and right ventricular hypertrophy
- Gastroschisis is a right paraumbilical defect with free-floating, uncovered bowel and no significant aneuploidy association, whereas omphalocele is a membrane-covered midline cord-insertion defect strongly associated with trisomies 18 and 13
- Posterior urethral valves produce the keyhole sign — a dilated bladder continuous with a dilated posterior urethra — with bilateral hydronephrosis and oligohydramnios in a male fetus
Central Nervous System Anomalies
Anencephaly is failure of the rostral neuropore to close, producing absence of the calvarium (skull vault) and cerebral hemispheres above the orbits. The prominent orbits with exposed disorganized tissue give the classic frog-eye appearance. Because the defect is open, maternal serum alpha-fetoprotein (MSAFP) is markedly elevated, and impaired swallowing causes polyhydramnios. Anencephaly is lethal and is considered the end stage of the acrania-exencephaly-anencephaly sequence.
Encephalocele is herniation of brain and meninges through a skull defect, most often occipital. It appears as a sac protruding from the cranium; when completely skin-covered, MSAFP may be normal.
Spina bifida (open neural tube defect of the spine) is usually detected first through two intracranial signs rather than the spine itself:
- Lemon sign — bilateral scalloping of the frontal bones, so the skull contour resembles a lemon
- Banana sign — the cerebellum is pulled down and curved around the brainstem (Arnold-Chiari II malformation), obliterating the cisterna magna
In the spine, look for splaying of the posterior ossification centers and a saccular myelomeningocele. Ventriculomegaly (atrial width > 10 mm) commonly coexists.
Holoprosencephaly is failed cleavage of the prosencephalon. The alobar form — the most severe — shows a single horseshoe-shaped ventricle, fused thalami, absent falx and cavum septi pellucidi, and midline facial defects; it is strongly associated with trisomy 13. Dandy-Walker malformation combines cystic dilation of the fourth ventricle with vermian agenesis and an elevated tentorium; milder posterior fossa variants include inferior vermian hypoplasia and mega cisterna magna (> 10 mm). Agenesis of the corpus callosum produces colpocephaly — teardrop-shaped ventricles with dilated occipital horns — an absent cavum septi pellucidi, and a high-riding third ventricle.
Cardiac and Thoracic Anomalies
The ventricular septal defect (VSD) is the most common congenital heart defect and the easiest to miss on a four-chamber view; color Doppler across the septum improves detection. Hypoplastic left heart syndrome shows a diminutive or absent left ventricle with mitral and/or aortic atresia. Tetralogy of Fallot has four components: a VSD, an overriding aorta, pulmonary stenosis, and right ventricular hypertrophy; sonographically the overriding aorta is the key clue on the outflow views. An echogenic intracardiac focus is not a structural defect at all but a soft aneuploidy marker.
In the chest, congenital diaphragmatic hernia — usually left-sided through the foramen of Bochdalek — lets abdominal contents into the thorax. The signature finding is the stomach bubble in the chest beside the heart, with mediastinal shift to the right and polyhydramnios; lung compression causes pulmonary hypoplasia.
| CNS anomaly | Signature finding |
|---|---|
| Anencephaly | Absent calvarium above the orbits, elevated MSAFP |
| Spina bifida | Lemon and banana signs, splayed posterior elements |
| Alobar holoprosencephaly | Single ventricle, fused thalami, absent falx |
| Dandy-Walker | Fourth-ventricle cyst, vermian agenesis |
| Agenesis of corpus callosum | Colpocephaly, absent cavum septi pellucidi |
Gastrointestinal Anomalies
Esophageal atresia, usually with a tracheoesophageal fistula, prevents the fetus from routing swallowed fluid to the stomach, so the stomach bubble is absent or persistently small and polyhydramnios develops. The combination of an absent stomach bubble plus polyhydramnios should trigger a careful search for this diagnosis.
Duodenal atresia produces the double-bubble sign: two fluid-filled structures in the upper abdomen representing the dilated stomach and the dilated proximal duodenum, with polyhydramnios. Its importance is the association — roughly 30% of fetuses with duodenal atresia have trisomy 21.
The two anterior abdominal wall defects are a favorite exam contrast:
| Feature | Gastroschisis | Omphalocele |
|---|---|---|
| Location | Right paraumbilical, beside the cord | Midline, at the cord insertion |
| Covering membrane | None — bowel floats freely | Yes — peritoneum/amnion sac |
| Contents | Bowel only (usually) | Bowel and often liver |
| Aneuploidy association | Not associated | Strongly associated (trisomy 18, 13) |
| MSAFP | Elevated | Elevated (less so if covered) |
| Other associations | Young maternal age, bowel damage | Beckwith-Wiedemann, cardiac defects |
Genitourinary Anomalies
Bilateral renal agenesis means no kidneys form; the renal fossae are empty and the flattened adrenal glands fill the space (the lying-down adrenal sign). Without fetal urine, severe oligohydramnios develops after about 16 weeks, producing the Potter sequence — pulmonary hypoplasia, limb contractures, and compressed facies — which is lethal.
Multicystic dysplastic kidney (MCDK) shows multiple non-communicating cysts of varying sizes replacing the kidney, with no normal parenchyma; it is usually unilateral, often involutes spontaneously, and is compatible with normal outcome if the other kidney is normal. Infantile (autosomal recessive) polycystic kidney disease instead causes bilaterally enlarged, diffusely echogenic kidneys with oligohydramnios.
Posterior urethral valves, seen in male fetuses, obstruct the outlet: the bladder is massively dilated (megacystis) and continuous with a dilated posterior urethra, forming the keyhole sign, accompanied by bilateral hydronephrosis and oligohydramnios.
Skeletal and Growth Anomalies
Macrocephaly — head circumference above the 95th percentile — prompts evaluation for hydrocephalus, hydranencephaly, intracranial masses, and overgrowth syndromes. Limb reduction defects range from absence of a single digit to amelia (complete absence of a limb) and phocomelia (hands or feet attached close to the trunk, historically linked to thalidomide). Club foot (talipes equinovarus) shows the foot fixed at a right angle to the lower leg in the same plane as the tibia and fibula; when isolated it is usually benign, but with other anomalies it raises the risk of aneuploidy.
During an 18-week scan, the cerebellum appears curved around the brainstem with an obliterated cisterna magna, and the frontal bones are scalloped. These findings should prompt a careful search for which abnormality?
A fetus has bowel loops floating freely in the amniotic fluid through a defect just to the right of a normally inserted umbilical cord. Which statement is correct?
A male fetus has a massively dilated bladder continuous with a dilated proximal urethra, bilateral hydronephrosis, and oligohydramnios. What is the most likely diagnosis?