A 28-year-old primigravida at 20 weeks gestation undergoes a routine anatomy ultrasound. The sonographer identifies that the umbilical cord contains only ONE umbilical artery instead of the normal two, as shown by the structure marked **D** in the diagram. Which of the following is the MOST appropriate next step in the antenatal management of this finding?

Explanation

## Why "Perform detailed fetal anatomy scan and fetal echocardiogram to screen for congenital anomalies and chromosomal abnormalities" is right Single umbilical artery (SUA) — the absence of one of the normally TWO umbilical arteries marked as **D** — is found in ~1% of pregnancies and is strongly associated with congenital anomalies (especially cardiac, renal, and GI defects in 30–50% of SUA cases vs. 1% in the general population) and chromosomal abnormalities, particularly trisomy 18. According to Williams Obstetrics 26e, the standard workup for prenatally diagnosed SUA includes detailed anatomy ultrasound and fetal echocardiography to detect structural and chromosomal abnormalities. This allows for appropriate counseling, planning of delivery at a tertiary center if needed, and postnatal management. ## Why each distractor is wrong - **"Reassure the mother that single umbilical artery is a benign variant with no clinical significance"**: This is dangerously incorrect. SUA is NOT benign; it carries a 30–50% risk of congenital anomalies and is associated with trisomy 18, IUGR, and preterm birth. Reassurance without investigation violates standard of care. - **"Proceed directly to amniocentesis for karyotyping without further imaging"**: While chromosomal abnormalities are a concern, the first step is detailed ultrasound imaging (anatomy scan and fetal echo) to identify structural anomalies. Invasive testing (amniocentesis) may be considered based on imaging findings and patient choice, but imaging must precede it. - **"Recommend termination of pregnancy given the high risk of fetal demise"**: SUA is not an indication for termination. Although it increases risk of adverse outcomes (IUGR, preterm birth, congenital anomalies), many pregnancies with SUA result in healthy neonates. Termination is only considered if a lethal anomaly or severe chromosomal abnormality is identified on detailed imaging. **High-Yield:** Single umbilical artery (one artery instead of two) = screen for cardiac/renal/GI anomalies and trisomy 18 with detailed anatomy scan and fetal echo; associated with 30–50% anomaly rate vs. 1% baseline. [cite: Williams Obstetrics 26e — Placental Anatomy and Umbilical Cord Abnormalities]