What are the key differential diagnoses for a 27‑week‑gestation pregnant woman presenting with isolated right‑flank pain without fever, dysuria, urgency, or signs of labor?

Differential Diagnoses for Isolated Right Flank Pain at 27 Weeks Gestation

The primary differential diagnoses are physiologic hydronephrosis of pregnancy (present in >80% of pregnancies at this gestational age), urolithiasis with pathologic obstruction, and pyelonephritis, with ultrasound as the mandatory first imaging study to distinguish between these entities. 1

Most Likely Diagnoses

Physiologic Hydronephrosis of Pregnancy

• Occurs in >80% of pregnancies after 20 weeks gestation, predominantly affects the right side, and results from uterine compression of the ureter at the linea terminalis combined with progesterone-mediated reduction in ureteric peristalsis. 2, 1
• This is the most common cause of right flank pain at 27 weeks gestation and is typically benign, though it can cause significant discomfort. 1
• The grade of hydronephrosis does not correlate with pain intensity, meaning severe pain can occur even with mild hydronephrosis. 3

Urolithiasis with Pathologic Obstruction

• Urolithiasis is the most common non-obstetric surgical emergency requiring intervention during pregnancy and can be superimposed on physiologic hydronephrosis. 1
• Ultrasound identifies hydronephrosis in approximately 84% of pregnant women with acute flank pain, but cannot reliably distinguish physiologic from pathologic obstruction. 1, 3
• The key differentiating feature is whether conservative management fails after 48-72 hours or if infection develops, which suggests pathologic obstruction requiring intervention. 1

Pyelonephritis

• Must be ruled out immediately through laboratory assessment including temperature, heart rate, complete blood count with differential, urinalysis with culture, and comprehensive metabolic panel. 1
• Fever, leukocytosis with left shift, or pyuria indicate infection and mandate urgent urinary decompression within 24 hours to prevent urosepsis and preterm labor. 1
• Can present with isolated flank pain before systemic symptoms develop, making early laboratory evaluation critical. 1

Less Common but Important Differentials

Appendicitis

• The appendix migrates superiorly with advancing gestation and can reach the right upper quadrant or right flank by the third trimester, potentially mimicking renal pathology. 4
• Appendicitis is the most common cause of abdominal pain requiring emergent surgery in pregnancy, with perforation carrying significant maternal and fetal morbidity. 5, 4
• The absence of fever, nausea, or gastrointestinal symptoms makes this less likely but does not exclude it. 5

Hepatobiliary Disease

• Acute cholecystitis or choledocholithiasis can present with right flank pain radiating from the right upper quadrant. 4
• More likely if pain is associated with nausea, vomiting, or postprandial exacerbation. 4

Preterm Labor

• Evaluation for preterm labor is essential, as flank pain from urolithiasis represents the most significant obstetric risk and typically resolves once the stone passes or is removed. 1
• Uterine contractions, cervical changes, or vaginal bleeding would suggest this diagnosis. 1

Diagnostic Algorithm

Initial Workup

• Obtain vital signs (temperature, heart rate, blood pressure), complete blood count with differential, comprehensive metabolic panel, and urinalysis with culture immediately to rule out infection. 1
• Perform renal and bladder ultrasound as the mandatory first imaging study because it avoids ionizing radiation, is highly sensitive for hydronephrosis, and can identify stones in many cases. 1

Second-Line Imaging

• If ultrasound is equivocal or non-diagnostic, proceed to MRI abdomen/pelvis without IV contrast or MR urography without contrast. 1
• MRI can differentiate physiologic from pathologic hydronephrosis and detect ureteral calculi while avoiding radiation, though its sensitivity for exact stone location is approximately 69% compared to CT. 2, 1, 6

Third-Line Imaging

• Reserve low-dose CT abdomen/pelvis without IV contrast for cases where both ultrasound and MRI are non-diagnostic or unavailable and clinical suspicion remains high. 1
• Modern low-dose protocols deliver an average fetal dose of approximately 705 mrads and retain high sensitivity and specificity for urinary calculi. 1, 7

Critical Red Flags Requiring Immediate Escalation

• Development of fever, tachycardia, leukocytosis, or any laboratory/clinical evidence of infection mandates urgent urinary decompression within 24 hours. 1
• Pain refractory to conservative therapy (rest, hydration, acetaminophen) after 48-72 hours indicates need for urologic intervention. 1
• Signs of preterm labor (contractions, cervical changes) despite conservative measures require immediate obstetric consultation. 1
• Hemodynamic instability or signs of sepsis require emergency intervention regardless of imaging findings. 5, 4