What is the appropriate workup for a pregnant woman presenting with right flank pain?

Workup for Right Flank Pain in Pregnancy

Begin with ultrasound as the mandatory first-line imaging study for all pregnant patients with right flank pain, followed immediately by MRI without IV contrast if ultrasound is non-diagnostic, reserving CT only for life-threatening situations where other modalities fail or are unavailable. 1, 2

Initial Clinical Assessment

Evaluate for specific red flags that guide urgency and differential diagnosis:

• Fever, tachycardia, or signs of sepsis suggest pyelonephritis or complicated urolithiasis requiring urgent intervention 1
• Severe unremitting pain with hemodynamic instability may indicate complications such as spontaneous adrenal hemorrhage, which is rare but life-threatening in pregnancy 3
• Associated symptoms including vaginal bleeding, syncope, or abdominal pain broaden the differential to include obstetric emergencies 2, 4
• History of prior kidney stones increases likelihood of recurrent urolithiasis, which occurs in approximately two-thirds of pregnant patients with nephrolithiasis 5

Imaging Algorithm

First-Line: Ultrasound (Rating 8 - Usually Appropriate)

Ultrasound is the initial imaging modality of choice because it avoids ionizing radiation to the fetus and has reasonably good sensitivity for stone detection and excellent ability to detect hydronephrosis. 1

• Ultrasound can identify hydronephrosis, which may be physiologic in pregnancy rather than pathologic, particularly on the right side due to dextrorotation of the gravid uterus 1
• The sensitivity for detecting stones is limited, but ultrasound reliably identifies complications such as perinephric edema and significant obstruction 1
• Physiologic hydronephrosis occurs commonly in pregnancy and should not automatically trigger intervention 1

Second-Line: MRI Without IV Contrast (Rating 5-6 - May Be Appropriate)

If ultrasound is non-diagnostic or equivocal, proceed immediately to MRI abdomen and pelvis without IV contrast, which provides superior diagnostic accuracy while avoiding radiation exposure. 1, 2

• MRI is highly dependable for depicting hydronephrosis and perinephric edema even when stone visualization is suboptimal 1
• The American College of Radiology recommends MRI in the second and third trimesters when renal calculus is suspected and ultrasound is inconclusive 1
• Gadolinium-based contrast should only be administered when potential significant benefit outweighs the unknown risk of fetal exposure to free gadolinium ions, requiring careful review by clinical and radiology teams 1

Third-Line: Low-Dose CT (Rating 6 - May Be Appropriate)

Reserve CT for situations where MRI is unavailable or results remain equivocal after both ultrasound and MRI, and when the clinical situation is potentially life-threatening. 1, 2

• Low-dose noncontrast CT has the same diagnostic effectiveness during pregnancy as in non-pregnant patients but carries radiation exposure concerns 1
• Average fetal radiation exposure with low-dose protocols is approximately 700 mrads (range 210-1372 mrads), which confers low risk of fetal harm when used judiciously 6
• Maternal health takes absolute priority - CT imaging should not be withheld if maternal survival is at risk 2
• Low-dose CT demonstrates high sensitivity and specificity for urinary calculi and can improve patient care in refractory cases 6

Differential Diagnosis Considerations

The right flank pain differential in pregnancy includes:

• Urolithiasis - most common cause of flank pain in pregnancy, with spontaneous stone passage occurring in approximately two-thirds of patients 5
• Pyelonephritis - particularly if fever and urinary symptoms are present 1, 5
• Physiologic hydronephrosis - common in pregnancy and may not require intervention 1
• Appendicitis - can present with right flank pain, particularly in later trimesters when the appendix is displaced superiorly 1
• Spontaneous adrenal hemorrhage - rare but life-threatening, may present with severe flank pain and develop into adrenal insufficiency 3

Management Principles

Do not delay imaging or intervention in pregnant patients with suspected complicated urolithiasis or infection, as complications including perforation or sepsis carry significant maternal and fetal morbidity and mortality. 2, 7

• Conservative management is appropriate for uncomplicated cases, as spontaneous stone passage occurs in most patients 5
• Retrograde ureteral stent placement is effective for therapy-resistant flank pain due to obstructive hydronephrosis, with low morbidity rates and can be performed under local anesthesia with sonographic guidance 7
• Immediate surgical or urologic consultation is warranted for hemodynamic instability, signs of sepsis, refractory pain despite conservative measures, or confirmed complicated urolithiasis 2, 7

Common Pitfalls

• Do not assume all hydronephrosis in pregnancy is pathologic - physiologic hydronephrosis is common and may not correlate with symptoms 1
• Do not withhold necessary imaging due to radiation concerns when clinical suspicion for serious pathology is high - maternal survival is the priority 2, 6
• Do not overlook rare but serious causes such as spontaneous adrenal hemorrhage, which may present similarly to urolithiasis but requires different management 3