What is the appropriate workup for a pregnant woman with a pelvic kidney?

Workup for Maternal Pelvic Kidney Discovered During Pregnancy

When a maternal pelvic kidney is incidentally discovered during pregnancy, the appropriate workup consists of ultrasound evaluation to assess renal anatomy, function, and associated anomalies, with no further imaging required if the kidney appears structurally normal and the patient is asymptomatic. 1

Initial Assessment Strategy

Clinical Evaluation

• Document whether the finding is symptomatic or incidental, as this fundamentally determines the workup intensity 2
• Assess for flank pain, urinary tract infection symptoms, hematuria, or signs of renal dysfunction 3
• Review for history of recurrent UTIs, stones, or prior renal issues 1

First-Line Imaging: Ultrasound with Color Doppler

Ultrasound with color Doppler of the kidneys and bladder is the definitive initial and often only necessary imaging modality 1:

• Confirm the pelvic location and document precise anatomical position relative to the bladder 4, 5
• Measure kidney size and compare to the contralateral kidney (pelvic kidneys typically have normal dimensions) 4, 5
• Assess for hydronephrosis by measuring anteroposterior diameter of the renal pelvis 1
• Evaluate for structural anomalies including multicystic dysplasia, stones, or masses 4, 5
• Measure resistive indices (RI) to assess renal perfusion and function—RI >0.70 suggests underlying dysfunction 1
• Examine the contralateral kidney for compensatory changes or additional anomalies 4, 5

Risk Stratification Based on Ultrasound Findings

Asymptomatic with Normal Ultrasound (Most Common Scenario)

• No further imaging is required if the pelvic kidney appears structurally normal without hydronephrosis or masses 4, 5
• Reassure the patient that normal renal function is highly probable and early intervention is unnecessary 4
• Schedule follow-up ultrasound every 6 months initially, then annually if stable 6

Symptomatic or Abnormal Ultrasound Findings

If ultrasound reveals concerning features (hydronephrosis >2 cm, stones, masses) or symptoms persist despite conservative management, proceed to MRI 1:

• MRI abdomen and pelvis without contrast is the second-line modality, avoiding fetal radiation exposure 1
• MRI can detect congenital anomalies, scarring, abscesses, and better characterize masses 1
• MR urography (MRU) without contrast can be added to evaluate the collecting system if obstruction is suspected 1
• Avoid gadolinium-based contrast unless the indication is critical and benefits clearly outweigh unknown fetal risks 1

Management of Specific Complications

Hydronephrosis in Pelvic Kidney

• Distinguish physiologic from pathologic hydronephrosis using RI measurements (RI difference >0.04 between kidneys suggests obstruction) 1
• Anteroposterior diameter thresholds for intervention: >16.5 mm in first/second trimester or >27.5 mm in third trimester 1
• Conservative management first with analgesics and hydration 2
• Consider double-J stent placement under ultrasound guidance if pain persists, progressive hydronephrosis develops, or uterine contractions occur 2

Suspected Stones or Infection

• Ultrasound has nearly 100% sensitivity for stones >5 mm but poor accuracy for stones <3 mm 1, 7
• If ultrasound is nondiagnostic and symptoms persist, proceed to MRU without contrast as second-line imaging 1, 7
• Low-dose noncontrast CT can be considered only when diagnosis remains uncertain after ultrasound and MRI, and clinical urgency exists (delivers <20 mGy to uterus) 7

Critical Pitfalls to Avoid

• Do not mistake pelvic kidney for renal agenesis—when a kidney is not visualized in the renal fossa, always scan the pelvis with a full bladder before concluding agenesis 6
• Do not attribute all hydronephrosis to physiologic pregnancy changes—physiologic hydronephrosis occurs in >80% of pregnant patients but pathologic obstruction must be excluded with RI measurements and clinical correlation 1, 8
• Avoid CT as initial imaging due to unnecessary fetal radiation exposure when ultrasound and MRI provide adequate diagnostic information 1, 7
• Do not use DMSA scans, VCUG, or IVU during pregnancy—these modalities provide no benefit and expose the fetus to radiation 1
• Recognize that most pelvic kidneys are diagnosed after 24 weeks gestation, so earlier negative scans do not exclude the diagnosis 4, 5

Functional Assessment

• Nuclear medicine scans (DMSA or MAG3) are reserved for postnatal evaluation if functional assessment is needed 6
• Overall renal function typically remains normal even when the pelvic kidney shows impaired partial function (mean 25-35% of total function) 5, 6
• Serious complications requiring nephrectomy are rare (approximately 3% risk of multicystic dysplasia) 5