Genitourinary System

Systematic evaluation of the fetal genitourinary system in a Level II scan includes both kidneys, ureters (when visible), bladder, amniotic fluid volume, and genitalia. Findings should be interpreted in the context of gestational age and overall fetal anatomy.

Kidneys

• Location:
  • By mid-gestation, both kidneys should be identified in the upper abdomen, adjacent to the spine.
  • Absence of a kidney from its usual fossa should prompt a search for an ectopic or pelvic kidney.
• Appearance:
  • Renal cortex slightly hypoechoic relative to liver; central echo complex corresponds to collecting system and vessels.
  • Kidney size increases with gestational age; overall shape is reniform (kidney-shaped).
• Pyramids & pelvis:
  • Mild visualization of the renal pelvis can be normal, especially later in gestation.
  • Calyces may be seen in some normal fetuses; assessment focuses on degree of dilatation.

Bladder

• Normally visualized as an anechoic, midline pelvic structure.
• Should fill and empty over time; a persistently absent bladder with severe oligohydramnios suggests bilateral renal agenesis or severe obstruction.
• Markedly enlarged bladder suggests megacystis or lower urinary tract obstruction (see LUTO section).

Ureters

• Typically not visible when normal.
• Visualized ureters are usually dilated, suggesting obstructive or reflux-related pathology (see Bladder & ureter anomalies).

Genitalia (overview)

• External genitalia are generally well visualized in the mid-second trimester with appropriate fetal position.
• Evaluation includes confirmation of expected sex (when appropriate), symmetry, and absence of masses.
• Ambiguous or discordant genitalia should prompt assessment for differences of sex development (DSD) and detailed anatomic survey.

Normal-appearing kidneys with an intermittently filling bladder and normal amniotic fluid significantly reduce the likelihood of severe renal dysplasia or bilateral obstruction, though mild or unilateral disease may still be present.

Quantitative assessment of the fetal genitourinary system focuses on renal pelvis diameter, kidney size, and bladder dimensions. These measurements are interpreted alongside amniotic fluid volume and gestational age.

Renal pelvis anteroposterior diameter (APD)

• Measure in the transverse plane at the level of the renal hilum, inner wall to inner wall.
• Thresholds vary slightly by guideline; a common pragmatic approach:
  • 2nd trimester (≈16–27 weeks):
    • APD < 4 mm: usually considered within normal limits.
    • APD 4–7 mm: mild pyelectasis / hydronephrosis.
    • APD > 7 mm: moderate or greater dilatation.
  • 3rd trimester (≥28 weeks):
    • APD < 7 mm: usually considered within normal limits.
    • APD 7–10 mm: mild to moderate dilatation.
    • APD > 10 mm: more significant hydronephrosis.

Kidney length

• Measured in the longest longitudinal axis for each kidney.
• Kidney length increases roughly in parallel with gestational age; Z-score calculators or charts are used to assess for small or enlarged kidneys.

Bladder size

• In routine practice, bladder assessment is primarily qualitative (normal vs small vs enlarged) and dynamic (presence of filling/emptying).
• Markedly enlarged bladder (> 7–8 mm in first trimester, or subjectively large later) may be described as megacystis and evaluated for LUTO.

Borderline renal pelvis measurements are common and frequently benign. Serial follow-up, association with other anomalies, and postnatal evaluation help distinguish physiologic dilatation from clinically significant hydronephrosis or obstructive uropathy.

Fetal renal pelvis dilatation (pyelectasis/hydronephrosis) is one of the most common findings on prenatal ultrasound. Most cases are mild and transient, but more significant dilatation may indicate obstruction, reflux, or underlying dysplasia.

Definitions & terminology

• Pyelectasis: mild dilatation of the renal pelvis, often used when APD is only slightly above normal thresholds.
• Hydronephrosis: more pronounced dilatation of the collecting system; often associated with calyceal dilatation and/or cortical changes.

Key sonographic features to document

• Laterality (unilateral vs bilateral).
• Renal pelvis AP diameter (per trimester thresholds).
• Presence and degree of calyceal dilatation.
• Cortical thickness and echogenicity.
• Ureteral visualization and bladder appearance.
• Amniotic fluid volume (normal vs oligohydramnios).

While slightly different schemes exist, many prenatal and postnatal systems now converge on a structured “UTD” (Urinary Tract Dilation) approach, incorporating APD, calyceal involvement, parenchymal appearance, ureter status, and bladder findings to stratify risk.

Isolated mild pyelectasis with otherwise normal anatomy and normal amniotic fluid is common and usually has a favorable outcome. More significant dilatation, bilateral involvement, cortical changes, or associated anomalies warrant closer follow-up and multidisciplinary planning.

Cystic renal diseases in the fetus include a spectrum from unilateral multicystic dysplastic kidney (MCDK) to bilateral autosomal recessive polycystic kidney disease (ARPKD) and obstructive dysplasia. The pattern of cysts, renal size, echogenicity, and amniotic fluid volume helps distinguish entities.

Key categories and patterns

• Multicystic dysplastic kidney (MCDK):
  • Unilateral or bilateral kidney replaced by multiple, noncommunicating cysts of varying size.
  • Little or no identifiable normal parenchyma; kidney often enlarged and lobulated in outline.
  • Contralateral kidney may be normal or show associated anomalies.
• Autosomal recessive polycystic kidney disease (ARPKD):
  • Enlarged, diffusely hyperechoic kidneys with loss of corticomedullary differentiation.
  • Cysts are typically microscopic and not individually resolved by ultrasound.
  • Often associated with oligohydramnios and risk of pulmonary hypoplasia.
• Autosomal dominant polycystic kidney disease (ADPKD):
  • May be subtle prenatally; kidneys can appear slightly enlarged and echogenic, sometimes normal.
  • Family history often key to diagnosis; many cases present postnatally or in adulthood.
• Obstructive cystic dysplasia:
  • Cortical cysts or dysplastic pattern in a kidney distal to chronic obstruction (e.g., severe ureteropelvic junction obstruction or reflux).

Bilateral severe cystic renal disease with oligohydramnios in mid-gestation often portends a guarded prognosis because of pulmonary hypoplasia and renal failure. Unilateral MCDK with a normal contralateral kidney typically has a more favorable outlook.

Abnormalities of renal number and position include unilateral or bilateral agenesis, ectopic pelvic kidney, and fusion anomalies such as horseshoe kidney. These findings should be correlated with bladder visibility and amniotic fluid volume.

Renal agenesis

• Bilateral renal agenesis:
  • Non-visualization of both kidneys and renal arteries.
  • Absent bladder filling.
  • Severe oligohydramnios after the first trimester.
  • Typically incompatible with postnatal survival due to pulmonary hypoplasia.
• Unilateral renal agenesis:
  • Non-visualization of one kidney with compensatory enlargement of the contralateral kidney.
  • Bladder and amniotic fluid volume generally normal.
  • Often compatible with a good outcome, though associated anomalies should be excluded.

Ectopic & fusion anomalies

• Pelvic kidney:
  • Kidney located in the pelvis rather than the usual lumbar position.
  • May be misinterpreted as absent if only the upper abdomen is surveyed—systematic scanning is important.
• Horseshoe kidney and other fusion anomalies:
  • Lower poles of kidneys may be fused across the midline, sometimes visible as a bridge of tissue.
  • Often associated with other anomalies or chromosomal differences in some series.

Any suspected agenesis or ectopic kidney should prompt careful assessment of the contralateral kidney, bladder, amniotic fluid, and associated anomalies, with consideration of postnatal renal imaging and long-term follow-up planning.

Lower urinary tract obstruction (LUTO) encompasses conditions that block urinary outflow at or near the bladder outlet, such as posterior urethral valves (PUV) and urethral atresia. These may lead to megacystis, hydronephrosis, oligohydramnios, and renal dysplasia.

Key sonographic features

• Megacystis:
  • Markedly enlarged bladder that fails to empty during observation.
  • In first trimester, a bladder longitudinal diameter ≥7 mm is often considered megacystis.
• “Keyhole” sign:
  • Dilated bladder with a projection suggestive of a dilated posterior urethra.
  • Often seen with posterior urethral valves.
• Upper tract changes:
  • Bilateral hydronephrosis and hydroureter.
  • Progressive cortical thinning or echogenicity may reflect renal damage.
• Amniotic fluid:
  • Oligohydramnios is concerning for significant impairment of renal function and lung development.

The prognosis in LUTO depends on the degree and duration of obstruction, renal parenchymal preservation, and amniotic fluid volume. Multidisciplinary counseling and careful follow-up are important, and some cases may be considered for fetal intervention in selected centers.

Abnormalities of the bladder and ureters may present as megacystis, bladder wall thickening, ureteral dilatation, or associated urinary ascites and urinoma.

Selected entities

• Megaureter:
  • Visible, dilated ureter(s), unilateral or bilateral.
  • May be obstructive or due to reflux; often associated with hydronephrosis.
• Ureterocele:
  • Cystic dilation of the distal ureter within the bladder; can be associated with duplex collecting system.
  • May appear as a thin-walled, intravesical cystic structure.
• Prune Belly (Eagle–Barrett) syndrome:
  • Triad of deficient abdominal wall musculature, urinary tract dilatation, and cryptorchidism in males.
  • Ultrasound findings may include massively dilated bladder and ureters, distended abdomen, and oligohydramnios.
• Urinary ascites:
  • Free fluid in the fetal abdomen due to urinary tract perforation (e.g., bladder rupture from severe obstruction).
  • Associated with markedly abnormal bladder or upper tract findings.

Bladder and ureteral abnormalities frequently overlap with LUTO and hydronephrosis. Detailed imaging of the entire urinary tract and amniotic fluid, plus postnatal urologic evaluation, are key to defining anatomy and management.

Evaluation of fetal genitalia is part of a comprehensive Level II examination. Beyond sex assignment, the goal is to identify major anomalies or discordance between phenotypic sex and other findings.

Normal appearance

• Female genitalia:
  • Labia and clitoris identified in sagittal and transverse planes; the “three-line” sign may be seen.
• Male genitalia:
  • Penis and scrotum visualized; testicular descent may not be complete prenatally.

Abnormal or ambiguous findings

• Apparent discordance between expected sex (from NIPT or chromosomes) and external genital appearance.
• Ambiguous genitalia (e.g., enlarged clitoris or small phallus, labioscrotal fusion, single urogenital sinus).
• Associated anomalies of the urinary tract, adrenal glands, or other organ systems.

Suspected differences of sex development (DSD) should prompt a detailed anatomic survey, careful review of prior testing, and coordination with genetics, endocrinology, and neonatology. Prenatal counseling focuses on diagnostic possibilities and postnatal evaluation plans rather than definitive sex assignment.