How should a 27‑week‑gestation pregnant woman with isolated right flank pain, without fever, dysuria, urgency, or signs of labor, be evaluated and managed?

Evaluation and Management of Right Flank Pain at 27 Weeks Gestation Without Fever or Urinary Symptoms

Begin with renal ultrasound as the initial imaging study, followed by conservative management with hydration, rest, and analgesia if physiologic hydronephrosis is confirmed without evidence of obstruction or infection. 1

Initial Diagnostic Approach

Start with renal and bladder ultrasound immediately, as this is the recommended first-line imaging modality for pregnant patients with flank pain because it avoids ionizing radiation, is sensitive for detecting hydronephrosis, and can identify stones in many cases. 1 Ultrasound is sufficient to confirm the diagnosis in 96% of pregnant patients with renal colic. 2

Key Clinical Assessment Points

• Assess for signs of infection or sepsis: Check temperature, heart rate, blood pressure, and obtain complete blood count with differential, urinalysis with culture, and comprehensive metabolic panel. 1 The absence of fever and urinary symptoms makes infection less likely but does not exclude it entirely.

• Evaluate for preterm labor: Right flank pain can trigger preterm labor, which is the most significant risk of urolithiasis during pregnancy and completely ceases once the stone passes or is removed. 1

• Consider physiologic hydronephrosis: Hydronephrosis occurs in >80% of pregnant patients after week 20, more commonly on the right side, due to compression of the ureter by the gravid uterus and hormonal effects reducing ureteric peristalsis. 1 At 27 weeks, physiologic hydronephrosis is extremely common and may be the sole cause of pain. 3

Imaging Algorithm

First-Line: Ultrasound

• Renal and bladder ultrasound will identify hydronephrosis (present in 84.3% of pregnant women with acute flank pain), detect stones if present, and assess for alternative diagnoses. 1, 3

Second-Line: MRI (If Ultrasound Non-Diagnostic)

• If ultrasound is equivocal or fails to explain symptoms, proceed to MRI abdomen/pelvis without IV contrast or MR urography without contrast. 1 MRI can detect ureteral calculi and other causes of obstruction while avoiding radiation exposure, though it has lower sensitivity (69%) than CT for identifying the exact site of stone impaction. 1

Third-Line: Low-Dose CT (Reserved for Specific Situations)

• CT abdomen/pelvis without IV contrast should only be used if both ultrasound and MRI are non-diagnostic or unavailable, and clinical suspicion for stone disease remains high. 1 Low-dose CT protocols (average 705 mrads) are highly sensitive and specific for detecting urinary calculi with acceptable fetal radiation exposure. 4

Management Strategy

Conservative Management (70-80% Success Rate)

If hydronephrosis is present without evidence of infection or severe obstruction, initiate medical management with:

• Adequate rest and hydration
• Antiemetics as needed
• Analgesia (acetaminophen preferred; avoid NSAIDs after 20 weeks)
• Close monitoring for development of fever, worsening pain, or signs of preterm labor 1

This approach succeeds in 70-80% of cases when stone disease is present without infection. 1

Indications for Urologic Intervention

Proceed to urinary decompression if:

• Pain remains refractory to conservative management after 48-72 hours
• Development of fever, leukocytosis, or signs of infection
• Evidence of significant obstruction with declining renal function
• Signs of preterm labor that do not resolve with conservative measures 1, 2

Retrograde ureteral stenting is the preferred intervention over percutaneous nephrostomy in pregnant patients, as it can be performed with minimal or no fluoroscopy using ultrasound guidance, has been demonstrated safe in all trimesters, and allows for definitive stone treatment with ureteroscopy if needed. 1 Percutaneous nephrostomy remains an alternative if retrograde stenting is not technically feasible. 1

Critical Pitfalls to Avoid

• Do not assume all hydronephrosis in pregnancy is physiologic: While common, pathologic obstruction from stones occurs in approximately 19.4% of pregnant patients presenting with flank pain. 2

• Do not delay imaging: Ultrasound should be performed promptly, as the grade of hydronephrosis does not correlate with pain intensity, and clinical assessment alone cannot distinguish physiologic from pathologic causes. 3

• Do not withhold intervention if infection develops: If fever or leukocytosis appears, this represents infected obstructed system requiring urgent decompression within 24 hours to prevent urosepsis and preterm labor. 1

• Monitor for rare complications: Spontaneous renal rupture, though extremely rare, can present with progressive flank pain in pregnancy and requires high clinical suspicion if pain escalates dramatically. 5

Expected Outcomes

Women with flank pain and hydronephrosis during pregnancy typically deliver at term (mean 38.1 weeks) with normal birthweight infants (mean 3138g) when appropriately managed. 3 The key is distinguishing physiologic from pathologic causes and intervening promptly when conservative management fails or complications develop.