Urologic Imaging

What Is the Safest Imaging Workup for Suspected Kidney Stones During Pregnancy?

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What Is the Safest Imaging Workup for Suspected Kidney Stones During Pregnancy?

A 32-year-old patient, 28 weeks pregnant, presents to the emergency department at 2 a.m. with acute, severe left flank pain radiating to the groin, accompanied by nausea. Her urinalysis shows microscopic hematuria. You strongly suspect obstructive urolithiasis, but the immediate question is how to confirm the diagnosis without harming the fetus. This clinical workflow details the evidence-based approach for this specific, high-stakes scenario. Based on the American College of Radiology (ACR) Appropriateness Criteria, the initial imaging study of choice, `US kidneys and bladder retroperitoneal`, is rated Usually Appropriate due to its diagnostic utility and, most critically, its lack of ionizing radiation.

Who Fits This Clinical Scenario?

This guidance is specifically for a pregnant patient at any gestational age who presents with acute onset flank pain where there is a clinical suspicion of stone disease (urolithiasis). This applies whether it is the patient’s first such episode or if she has a history of stones but is presenting with a new, acute event requiring evaluation.

This workflow is distinct from other similar, but clinically different, scenarios. This article does not apply if:

  • The patient is not pregnant. The imaging algorithm for non-pregnant adults is different, prioritizing low-dose non-contrast CT. For that scenario, refer to the guidance on initial imaging for a patient with no history of stone disease.
  • The patient has a known, current stone and recurrent symptoms. If a stone has already been diagnosed on recent imaging and the patient’s symptoms are consistent with that known stone, the clinical question may shift from diagnosis to assessing for new complications, which can alter the imaging choice.
  • A prior non-contrast CT was inconclusive. This specific situation, where a CT has already been performed and failed to provide a clear answer, triggers a different problem-solving algorithm.

Correctly identifying your patient’s specific clinical context is the first step to selecting the most appropriate and safest imaging study.

What Diagnoses Are You Working Up in This Scenario?

When a pregnant patient presents with acute flank pain, the differential diagnosis is focused but includes critical mimics that imaging must help differentiate. The primary goal is to identify or exclude conditions requiring urgent urologic or obstetric intervention.

Obstructive Urolithiasis
This is the leading concern. A kidney stone (calculus) that has passed into the ureter can cause a partial or complete blockage, leading to dilation of the upstream collecting system (hydronephrosis) and severe pain. The risk of urosepsis is significantly elevated if the obstructed urine becomes infected, posing a threat to both mother and fetus.

Physiologic Hydronephrosis of Pregnancy
This is the most common mimic and a frequent confounding factor. The gravid uterus can compress the right ureter (more commonly than the left) against the pelvic brim, causing mild to moderate hydronephrosis. This is a normal physiologic change, but it can be difficult to distinguish from a true pathologic obstruction on imaging, especially if mild.

Pyelonephritis
A kidney infection can present with flank pain, fever, and constitutional symptoms. While often a clinical diagnosis, imaging is crucial if the patient does not respond to antibiotics or if there is a concern for an underlying abscess or an obstructing stone causing a secondary infection (pyonephrosis), which is a urologic emergency.

Non-Urologic Causes
Less common but critical to consider are other causes of acute abdominal or flank pain in pregnancy. These include appendicitis (the appendix is often displaced superiorly and laterally by the uterus), ovarian torsion, degenerating uterine fibroids, or, in severe cases, obstetric emergencies like placental abruption.

Why Is Ultrasound of the Kidneys and Bladder the Recommended Study?

For a pregnant patient with suspected urolithiasis, the ACR designates `US kidneys and bladder retroperitoneal` as Usually Appropriate. The rationale is rooted in a safety-first principle, balancing diagnostic yield with the absolute priority of avoiding fetal radiation exposure.

The primary strength of ultrasound in this setting is its complete lack of ionizing radiation (0 mSv). It is safe at any stage of pregnancy. Ultrasound is highly sensitive for detecting hydronephrosis, the key secondary sign of a significant ureteral obstruction. While it may not always visualize the stone itself, especially in the mid-ureter, the presence of moderate to severe hydronephrosis in a symptomatic patient is highly suggestive of a pathologic obstruction.

Ultrasound can also help evaluate for other etiologies. It can assess for signs of pyelonephritis (e.g., altered renal echotexture) and can sometimes identify other pelvic pathology. A sonographer can also assess for ureteral jets—the periodic efflux of urine from the ureteral orifices into the bladder. The presence of symmetric ureteral jets makes a high-grade obstruction much less likely.

Why are alternative studies rated lower for this initial evaluation?

  • CT abdomen and pelvis without IV contrast: While the gold standard in non-pregnant patients, this study is only rated May be appropriate in pregnancy. The reason is its use of ionizing radiation (ACR estimate: ☢☢☢ 1-10 mSv), which carries a potential, albeit small, risk to the fetus. It is reserved as a second- or third-line option when ultrasound and/or MRI are non-diagnostic and the clinical risk of missing a diagnosis is high.
  • Radiography abdomen and pelvis (KUB): This is rated Usually not appropriate. It involves ionizing radiation (☢☢☢ 1-10 mSv) and has very low sensitivity for detecting both stones (many are not radiopaque) and the secondary signs of obstruction like hydronephrosis.
  • MR Urography (MRU) without IV contrast: Rated May be appropriate, this is an excellent radiation-free problem-solving tool. It is superior to ultrasound for visualizing the full length of the ureters and determining the precise level of obstruction. However, it is more costly, less readily available, and more time-consuming than ultrasound, making US the better initial screening test.

What’s Next After Ultrasound? Downstream Workflow

The results of the initial renal and bladder ultrasound will guide your next steps. The management pathway depends on whether the findings are positive, negative, or equivocal, always in the context of the patient’s clinical status.

If the ultrasound is POSITIVE (e.g., moderate-to-severe hydronephrosis):
A finding of significant hydronephrosis, especially if a stone is visualized in the renal pelvis or at the ureterovesical junction (UVJ), confirms the diagnosis of obstruction. The next step is an urgent consultation with both Urology and Obstetrics. Management may include pain control and trial of passage, or it may require intervention such as a ureteral stent or nephrostomy tube placement to decompress the collecting system, particularly if there are signs of infection or intractable pain.

If the ultrasound is NEGATIVE (no hydronephrosis, no stone seen):
If the patient’s symptoms are severe and persistent despite a negative ultrasound, a significant obstruction remains a possibility. A small stone may be obstructing without causing immediate, significant dilation. In this case, the next appropriate step is often an `MRU without IV contrast`. This radiation-free study can definitively assess for hydroureteronephrosis and identify the transition point, even if the stone itself isn’t seen.

If the ultrasound is INDETERMINATE (e.g., mild hydronephrosis):
This is the most common clinical challenge, as it can be difficult to distinguish physiologic hydronephrosis of pregnancy from a mild pathologic obstruction. If the patient is clinically stable, a period of observation with hydration and pain management is reasonable. If symptoms persist or worsen, or if there is any concern for infection, `MRU without IV contrast` is the ideal next step to clarify the anatomy and rule out a true obstruction.

Pitfalls to Avoid (and When to Get Help)

Navigating this scenario requires careful consideration to avoid common diagnostic and management errors.

  • Attributing all hydronephrosis to pregnancy: While physiologic hydronephrosis is common (especially on the right), do not automatically dismiss it. Correlate the degree of dilation with the severity of the patient’s symptoms. Asymmetric or severe hydronephrosis should raise suspicion for a pathologic cause.
  • Relying solely on direct stone visualization: The most important finding on ultrasound is often hydronephrosis, not the stone itself. A non-distended collecting system makes a clinically significant obstruction unlikely, even if you can’t find a stone.
  • Prematurely ordering a CT scan: The principle of ALARA (As Low As Reasonably Achievable) is paramount for fetal radiation safety. Exhaust non-ionizing modalities like repeat ultrasound or MRU before considering a low-dose CT scan.
  • Forgetting the non-urologic differential: If the renal ultrasound is completely normal but the patient’s pain is severe, pivot your workup. Re-examine the patient and consider imaging targeted at other possibilities, such as a dedicated appendix ultrasound or pelvic ultrasound to evaluate the ovaries.

If the diagnosis remains unclear after initial imaging or if the patient shows signs of sepsis (fever, tachycardia, hypotension), escalate immediately with concurrent consultations to Urology, Obstetrics, and Radiology.

Related ACR Topics and Tools

This article covers one specific scenario in depth. For a comprehensive overview of all clinical variants and imaging modalities for this condition, please consult the parent topic guide. For additional decision support, the following tools can help you apply appropriateness criteria and understand imaging protocols.

Frequently Asked Questions

Is it ever appropriate to order a CT scan as the first imaging test for a pregnant patient with flank pain?

Almost never. According to the ACR, ultrasound is the ‘Usually Appropriate’ initial test due to the lack of ionizing radiation. A low-dose non-contrast CT is rated ‘May be appropriate’ and is reserved for complex cases where ultrasound and/or MRI are non-diagnostic and the clinical suspicion for an urgent condition remains very high. The decision should be made in consultation with the radiologist and obstetrician.

What if the ultrasound shows mild right hydronephrosis but the patient’s pain is on the left?

This is a red flag that the mild right hydronephrosis is likely physiologic and unrelated to the patient’s acute symptoms. The left-sided pain requires further investigation. If the left kidney and ureter appear normal on ultrasound, an MR Urography without contrast would be the logical next step to evaluate the left ureter for a non-dilated or subtle obstruction.

Can Doppler ultrasound help differentiate physiologic from obstructive hydronephrosis?

Yes, it can be a useful adjunct. The sonographer can use color Doppler to look for ‘ureteral jets’—the small spurts of urine entering the bladder from the ureters. The presence of a strong, symmetric jet from the symptomatic side makes a high-grade obstruction less likely. However, the absence of a jet is not specific and can be seen in both obstruction and low-flow states. The ACR rates ‘US color Doppler kidneys and bladder retroperitoneal’ as ‘May be appropriate (Disagreement)’.

Is gadolinium-based contrast safe for an MRI/MRU during pregnancy?

Gadolinium-based contrast agents are generally avoided during pregnancy unless the potential benefit unequivocally outweighs the potential fetal risk. They can cross the placenta and enter the fetal circulation, with theoretical concerns about long-term retention. For this reason, the ACR rates MRI and MRU with contrast as ‘Usually not appropriate’ for this indication. A non-contrast MRU is sufficient for diagnosing hydronephrosis and the level of obstruction.

How does the patient’s gestational age affect the imaging choice?

The primary imaging algorithm does not change based on gestational age; ultrasound remains the first-line study throughout pregnancy because it is always radiation-free. However, gestational age can influence the interpretation of results (e.g., physiologic hydronephrosis is more common in the third trimester) and the management options available if an obstructing stone is found.

Reviewed by Pouyan Golshani, MD, Interventional Radiologist — May 30, 2026