Mild dilation of the renal pelvis without visible calyceal involvement
Significant dilation involving renal pelvis AND calyces
Note: Terms often used interchangeably in literature; focus on measurement and grading rather than terminology
Most common abnormality detected on antenatal ultrasound
Prevalence: 1-2% of all pregnancies
Clinical Implications:
Outer echogenic rim
Isoechoic to liver/spleen
Central anechoic collecting system
Should be minimal or not visible in 2nd trimester
Hypoechoic triangular structures
Do NOT mistake for dilated calyces
Not normally visible until dilated
Cup-shaped anechoic spaces when enlarged
Optimize fetal position for kidney visualization
Obtain transverse section through kidney at level of renal pelvis
Ensure true transverse plane—avoid oblique angles
Critical: Longitudinal measurements are NOT standardized and should not be used
Gold standard measurement for quantifying renal pelvic dilation
• Place calipers at inner-to-inner wall of renal pelvis
• Measure in anteroposterior dimension
• Perpendicular to long axis of renal pelvis
• At point of maximum dilation
Key: Inner-to-inner wall (not outer-to-outer)
| Gestational Age | Normal | Mild (Follow-up) | Moderate-Severe (Refer) |
|---|---|---|---|
| 16-27 weeks | < 4 mm | 4-7 mm | ≥ 7 mm |
| 28 weeks - term | < 7 mm | 7-10 mm | ≥ 10 mm |
Reference: Society for Fetal Urology (SFU) / Urinary Tract Dilation (UTD) Classification System
APRPD: 4-7 mm (< 28 wks) or 7-10 mm (≥ 28 wks)
Calyces: Normal
Parenchyma: Normal
APRPD: ≥ 7 mm (< 28 wks) or ≥ 10 mm (≥ 28 wks)
Calyces: Central or ≥ 1 peripheral
Parenchyma: Normal
APRPD: ≥ 10 mm (< 28 wks) or ≥ 15 mm (≥ 28 wks)
Calyces: Dilated peripheral
Parenchyma: Thinned or abnormal
Visible anechoic spaces adjacent to renal pelvis but not extending to cortex
Dilated cup-shaped structures reaching the corticomedullary junction
Significance: Peripheral calyceal dilation indicates more severe hydronephrosis and higher likelihood of pathology
Red Flag: Increased cortical echogenicity or thinned parenchyma suggests dysplasia or chronic obstruction
Most common scenario (80%)
Usually physiologic or UPJ obstruction
Lower risk of chromosomal abnormality
May resolve spontaneously
Higher clinical concern
Consider posterior urethral valves (males)
Higher association with chromosomal abnormalities
Assess bladder and amniotic fluid volume
Action: Bilateral findings warrant detailed anatomic survey and consider genetic counseling referral
Concerning: Persistently distended bladder + bilateral hydronephrosis + oligohydramnios = suspect lower urinary tract obstruction
Amniotic fluid volume reflects fetal renal function after 16-20 weeks
Suggests preserved renal function
Favorable prognosis even with hydronephrosis
Indicates significant bilateral renal dysfunction
Requires urgent MFM referral
Document: Single deepest pocket (SDP) or amniotic fluid index (AFI)
| Severity | Follow-up Interval | Action |
|---|---|---|
| Mild (UTD A1) | Reassess at 32 weeks | Routine follow-up |
| Moderate (UTD A2-1) | Every 4-6 weeks | MFM consultation |
| Severe (UTD A2-2/A3) | Every 2-4 weeks | Urgent MFM referral + postnatal urology |
Well-hydrated maternal state may transiently increase renal pelvic diameter; ideally measure after normal fluid intake
Moderately full maternal bladder improves visualization without causing fetal bladder overdistension
Renal pelvis diameter may vary throughout day; consistent timing improves serial comparison
Prominent medullary pyramids can mimic hydronephrosis
Hypoechoic (not anechoic)
Triangular shape
Do not communicate with renal pelvis
Symmetrically arranged
Anechoic (fluid-filled)
Rounded/cup-shaped
Continuous with renal pelvis
May be asymmetric
Normal anatomic variant where renal pelvis lies outside renal sinus
Management: Document as extrarenal pelvis; use caution interpreting isolated APRPD measurement
Example 1 (Mild):
"Mild left-sided pyelectasis with APRPD measuring 6 mm at 24 weeks gestation. No calyceal dilation. Normal renal parenchyma. Normal amniotic fluid volume. Recommend follow-up ultrasound at 32 weeks."
Example 2 (Severe):
"Bilateral severe hydronephrosis. Right APRPD 12 mm, left APRPD 14 mm at 30 weeks gestation. Peripheral calyceal dilation bilaterally. Renal parenchyma appears thinned. Distended bladder visualized. Amniotic fluid volume normal. MFM referral recommended."
Accurate sonographic diagnosis of pyelectasis and hydronephrosis requires:
Standardized measurement technique
Consistent imaging planes
Awareness of pitfalls
Gestational age-appropriate thresholds
Comprehensive urinary tract evaluation
Appropriate referral and follow-up
Standardized approach improves diagnostic accuracy and patient outcomes