A 20-week obstetric ultrasound is performed on a primigravida. The sonographer identifies a fluid-filled structure in the fetal abdomen that communicates with the umbilicus. Which single imaging feature best distinguishes an omphalocele from a gastroschisis?

## Omphalocele vs Gastroschisis: Key Discriminating Feature **Key Point:** The presence or absence of a membranous covering is the single most reliable ultrasound feature that distinguishes omphalocele from gastroschisis. ### Comparative Table | Feature | Omphalocele | Gastroschisis | | --- | --- | --- | | **Covering membrane** | **Present (sac)** | **Absent** | | Location | At umbilicus | Lateral to umbilicus (usually right) | | Contents | Bowel, liver, spleen | Bowel only | | Associated anomalies | 50–70% (Beckwith–Wiedemann, trisomy 13/18) | <10% (isolated defect) | | Bowel appearance | Protected, normal | Thickened, matted, edematous | | Prognosis | Better (covered) | Worse (exposed, inflamed) | **High-Yield:** On ultrasound, omphalocele shows a **sac-like protrusion with a thin echogenic rim** (the membrane); gastroschisis shows **free-floating bowel loops with no covering**. ### Clinical Pearl **Mnemonic — "O for Organ-covered":** Omphalocele = Organ + covering membrane. Gastroschisis = Gastro (stomach/bowel) + schisis (split/defect) = exposed bowel. The membrane in omphalocele is composed of peritoneum and amnion, which protects the viscera and allows delayed closure. In gastroschisis, the defect is in the fascia only, and exposed bowel becomes inflamed and thickened in utero. ### Why This Matters Clinically - **Omphalocele:** Often associated with genetic syndromes → requires fetal karyotyping and detailed anatomy scan. - **Gastroschisis:** Isolated defect in most cases → prognosis depends on bowel damage and need for staged surgical closure. [cite:Rumack Diagnostic Ultrasound 4e Ch 39]