Why are fetal collecting‑system (kidney‑ureter) anomalies usually not detected by obstetricians on routine prenatal ultrasound?

Why Collecting System Anomalies Are Missed on Prenatal Ultrasound

Many fetal collecting system anomalies—including double collecting systems, vesicoureteral reflux (VUR), and mild-to-moderate hydronephrosis—are not detected on routine prenatal ultrasound because they either develop or become apparent only after birth when urine flow increases, or because prenatal ultrasound has inherent limitations in detecting functional and subtle structural abnormalities of the urinary tract. 1

Key Reasons for Non-Detection

Timing and Physiologic Limitations

• Fetal urine production begins around 14 weeks' gestational age, and the kidneys and bladder become reliably visible by the end of the first trimester, but many collecting system abnormalities require higher urine flow rates to manifest as visible dilation 1

• The standard screening ultrasound is performed at 18–22 weeks gestation, which may be too early to detect mild or moderate collecting system dilation that becomes apparent only in the third trimester or postnatally 1, 2

• Normal prenatal anteroposterior renal pelvis diameter (APD) thresholds are <4 mm before 28 weeks and <7 mm after 28 weeks, meaning mild dilation below these cutoffs is considered physiologic and not reported 1, 2

High False-Negative Rates for Specific Conditions

• Vesicoureteral reflux (VUR) is poorly detected prenatally, with false-negative rates ranging from 40% to 90% across multiple studies, because VUR is a functional abnormality that does not consistently produce visible structural changes on static imaging 1

• Double collecting systems (duplex kidneys) were missed in 100% of cases in one study series, as these subtle anatomic variants may not produce obvious dilation or structural distortion visible on prenatal ultrasound 1

• Among children with abnormal post-UTI ultrasound findings, 78% (7 of 9 cases) had completely normal prenatal ultrasounds, including cases of hydronephrosis, moderate dilation, and double collecting systems 1

Structural vs. Functional Abnormalities

• Prenatal ultrasound excels at detecting severe structural obstructions (posterior urethral valves, severe ureteropelvic junction obstruction with APD ≥15 mm) but performs poorly for functional disorders like VUR or mild obstruction 1, 2

• Collecting system abnormalities without significant dilation—such as non-obstructive duplex systems, mild VUR, or early-stage megaureter—produce minimal or no visible changes on prenatal imaging 2, 3

Evolution of Pathology Over Time

• Many collecting system abnormalities worsen or become apparent between the second and third trimesters, and some only manifest postnatally when increased urine output stresses the system 1, 2

• Up to 45% of initially normal postnatal ultrasounds show abnormalities on repeat imaging at 1–6 months, demonstrating that even immediate postnatal studies can miss pathology that evolves over time 4

Clinical Implications

Why Postnatal Imaging Remains Essential

• Normal prenatal ultrasound does not exclude urinary tract pathology, and the American College of Radiology recommends postnatal renal and bladder ultrasound for any infant with antenatal urinary tract dilation, even if mild 1, 2, 4

• Postnatal ultrasound timing is risk-stratified: severe bilateral hydronephrosis or oligohydramnios warrants imaging within 48 hours, while isolated mild dilation can be assessed at 1–6 months 2, 4

• Follow-up ultrasound at 1–6 months is mandatory even when initial postnatal imaging is normal, as many abnormalities only become apparent with maturation of the urinary system 2, 4

Common Pitfalls to Avoid

• Do not assume normal prenatal ultrasound excludes collecting system pathology—maintain a high index of suspicion for VUR, duplex systems, and mild obstruction in any infant presenting with febrile UTI, regardless of prenatal imaging 1, 2

• Do not delay postnatal imaging in high-risk scenarios (severe bilateral hydronephrosis, oligohydramnios, bladder abnormalities), as conditions like posterior urethral valves require urgent diagnosis and intervention 2, 4

• Recognize that prenatal ultrasound sensitivity is approximately 91% for urinary tract dilation overall, but this figure drops dramatically for specific entities like VUR (10–60% detection) and non-obstructive duplex systems 1, 2

Role of Advanced Imaging

• Voiding cystourethrography (VCUG) is required postnatally to detect VUR in infants with bilateral high-grade hydronephrosis, duplex kidneys, ureterocele, abnormal bladder appearance, or febrile UTI history—none of which are reliably diagnosed prenatally 1, 2

• MAG3 renal scintigraphy at ≥2 months of age assesses differential renal function and drainage in cases of persistent or worsening hydronephrosis, providing functional information unavailable from prenatal ultrasound 1, 2, 4

Summary Algorithm

For any infant with prenatal urinary tract findings:

1. Perform postnatal renal and bladder ultrasound at timing determined by severity (within 48 hours for severe/bilateral findings or oligohydramnios; at 1–6 months for mild isolated dilation) 2, 4

2. Obtain VCUG if postnatal ultrasound shows bilateral high-grade hydronephrosis, duplex kidney, ureterocele, bladder abnormality, or if febrile UTI occurs 1, 2

3. Repeat ultrasound at 1–6 months even if initial postnatal study is normal, as 45% of abnormalities emerge only on follow-up imaging 4

4. Proceed to MAG3 scan if hydronephrosis persists/worsens or renal parenchymal thinning develops, to guide surgical decision-making 1, 2, 4