What is the significance of unilateral hydronephrosis (swelling of the kidney due to urine accumulation) in a fetal ultrasound (US) during the 3rd trimester?

Unilateral Hydronephrosis in Third Trimester Fetal Ultrasound

Unilateral hydronephrosis detected in the third trimester is most commonly a benign, transient finding that resolves spontaneously in approximately 80% of cases, but requires postnatal follow-up ultrasound to exclude pathologic obstruction, particularly when the anteroposterior diameter (APD) exceeds 7 mm. 1

Clinical Significance and Risk Stratification

The significance of third-trimester unilateral hydronephrosis depends critically on the degree of dilation and associated findings:

Low-Risk Features (UTD A1 Classification)

• APD <7 mm in the third trimester represents normal physiologic variation and requires only a single follow-up ultrasound at ≥32 weeks of gestation. 1
• Approximately 80% of cases with APD between 4-7 mm in the second trimester resolve completely without intervention. 1
• Isolated mild hydronephrosis carries minimal risk for aneuploidy (positive likelihood ratio of only 1.5 for trisomy 21). 1

Higher-Risk Features Requiring Closer Surveillance

• APD ≥7 mm in the third trimester warrants postnatal follow-up, as this threshold has 100% sensitivity for predicting unfavorable outcomes. 2
• The presence of calyceal dilation, abnormal parenchymal thickness or appearance, ureteral dilation, bladder abnormalities, or oligohydramnios elevates the classification to UTD A2-3, indicating increased risk. 1
• Rates of postnatal urinary tract pathology increase progressively: 14% for APD ≤7 mm, 27% for APD 8-15 mm, and 53% for APD >15 mm. 2

Differential Diagnosis

The most common etiologies in order of frequency are:

• Transient physiologic hydronephrosis (most common, resolves spontaneously) 3
• Pelviureteric junction (PUJ) obstruction (most common pathologic cause) 3, 4
• Vesicoureteric reflux (VUR) 4
• Vesicoureteric junction obstruction 4
• Megaureter 5

Antenatal Management Algorithm

For Isolated UTD A1 (APD <7 mm, no other abnormalities):

• Perform single follow-up ultrasound at ≥32 weeks of gestation to reassess. 1
• No aneuploidy screening is required if prior screening was negative or low-risk. 1
• If no prior aneuploidy screening was performed, counsel regarding the minimal increased risk (LR 1.5) and offer noninvasive screening with cell-free DNA or quad screen. 1

For UTD A2-3 or APD ≥7 mm:

• Perform comprehensive assessment including evaluation of calyceal dilation, parenchymal thickness, ureteral dilation, bladder morphology, and amniotic fluid volume. 1
• Assess for associated structural abnormalities, particularly cardiovascular and renal anomalies, as these increase aneuploidy risk from 4% to 50%. 1
• Serial ultrasound monitoring through the third trimester to track progression or resolution. 4, 2

Postnatal Management

Timing of Initial Postnatal Ultrasound:

• All infants with antenatal hydronephrosis require postnatal ultrasound confirmation, ideally performed after 48-72 hours of life to avoid false-negative results from physiologic oliguria in the first 24-48 hours. 4

Intensity of Postnatal Evaluation Based on APD:

• APD <10 mm postnatally: Observation with repeat ultrasound at 1 and 6 months. 4
• APD ≥10 mm and/or Society for Fetal Urology (SFU) grade 3-4: Screen for obstruction with voiding cystourethrography (VCUG) to exclude VUR and diuretic renography (MAG3 or DTPA scan) to assess for obstruction. 4

Indications for Surgical Intervention:

• Progressive increase in renal pelvic APD on serial ultrasounds 4
• Obstructed renogram pattern with differential renal function <35-40% or subsequent decline in function 4
• Development of urinary tract infections despite prophylaxis 4

Prognosis

• Approximately 45-50% of antenatal hydronephrosis cases resolve completely by the third trimester without any intervention. 4
• Among cases persisting to term, the majority (>70%) have favorable postnatal outcomes with either spontaneous resolution or stable mild hydronephrosis not requiring surgery. 6, 2
• When fetal renal pelvis measures <15 mm on prenatal ultrasound, progression to severe hydronephrosis requiring surgery is rare. 6
• Male fetuses have higher incidence, and left-sided involvement is more common than right-sided. 6

Critical Pitfalls to Avoid

• Do not dismiss APD ≥7 mm as purely physiologic—these cases require documented postnatal follow-up to exclude pathology. 2
• Do not perform postnatal ultrasound in the first 24-48 hours of life, as physiologic oliguria can mask significant hydronephrosis. 4
• Do not assume bilateral involvement or oligohydramnios is benign—these findings suggest significant pathology and warrant immediate comprehensive evaluation. 4
• Do not delay VCUG and renography in infants with postnatal APD ≥10 mm or SFU grade 3-4, as early detection of VUR allows for prophylactic antibiotics to prevent pyelonephritis and renal scarring. 4