Urologic Imaging

Should You Order Ultrasound for Microhematuria During Pregnancy? An ACR-Guided Workflow

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Should You Order Ultrasound for Microhematuria During Pregnancy? An ACR-Guided Workflow

It’s a routine second-trimester prenatal visit. Your patient, a 30-year-old G2P1 at 24 weeks, is doing well, but a dipstick urinalysis followed by microscopy reveals persistent microhematuria. She is asymptomatic, with no flank pain, dysuria, or fever. You need to decide if imaging is warranted and, if so, which study balances diagnostic yield with fetal safety. This is a common scenario where the need to evaluate the maternal urinary tract conflicts with the imperative to avoid fetal harm. This article provides a detailed clinical workflow for this specific presentation, clarifying the American College of Radiology (ACR) recommendations. For this patient, the initial imaging study, `US kidneys and bladder retroperitoneal`, is rated Usually Appropriate.

Who Fits This Clinical Scenario?

This guidance applies to a specific patient population: pregnant individuals at any gestational age who are found to have microscopic hematuria. Microhematuria is typically defined as three or more red blood cells per high-power field on a properly collected urine specimen, in the absence of an obvious benign cause. The finding may be incidental on a screening urinalysis or part of an evaluation for nonspecific symptoms.

This workflow is distinct from other clinical presentations. It does not apply to:

  • Patients with gross hematuria: The presence of visible blood in the urine is a separate ACR scenario that often warrants a more urgent and potentially different imaging approach, even in pregnancy.
  • Non-pregnant patients with microhematuria: Patients without the constraint of pregnancy, especially those with risk factors for malignancy (e.g., age >35, smoking history), follow a different diagnostic algorithm that frequently involves CT Urography.
  • Patients with a clear, resolving urinary tract infection (UTI): If microhematuria is discovered alongside symptoms of a UTI and resolves completely after appropriate antibiotic treatment, imaging may not be necessary unless there are signs of a complicated infection like pyelonephritis.

The focus here is solely on the initial imaging decision for a pregnant patient with isolated, asymptomatic, or minimally symptomatic microhematuria.

What Diagnoses Are You Working Up in This Scenario?

In a pregnant patient with microhematuria, the differential diagnosis is weighted toward benign and pregnancy-related conditions, while still considering less common but more serious pathology. The goal of imaging is to differentiate these possibilities safely.

Physiologic Hydronephrosis of Pregnancy: This is the most common anatomic finding in the urinary tract during pregnancy and a frequent cause of incidental findings. Progesterone-induced smooth muscle relaxation and mechanical compression of the ureters by the gravid uterus (more pronounced on the right side) lead to dilation of the collecting systems. While typically benign, it’s crucial to distinguish this from a pathologic obstruction.

Nephrolithiasis (Urolithiasis): Kidney stones are a primary concern. While the incidence is not necessarily increased in pregnancy, the diagnosis and management are more challenging. An obstructing stone can cause significant pain, predispose to infection, and threaten pregnancy outcomes. Imaging is key to identifying the stone’s size and location and assessing for associated obstruction.

Complicated Urinary Tract Infection (Pyelonephritis): While many UTIs are uncomplicated, microhematuria can be a sign of upper tract involvement. In patients with fever, flank pain, or other signs of pyelonephritis, imaging helps rule out an abscess or an underlying obstructive cause like a stone that requires intervention.

Renal Mass: Though uncommon in this age group, underlying renal pathology such as an angiomyolipoma (which can be prone to bleeding) or, very rarely, a renal cell carcinoma must be considered. Imaging can detect solid or complex cystic masses that require further characterization, often postpartum.

Why Is US kidneys and bladder retroperitoneal the Recommended Study?

The ACR designates `US kidneys and bladder retroperitoneal` as Usually Appropriate for the initial evaluation of microhematuria in pregnancy because it provides the best balance of diagnostic utility and maternal-fetal safety in this specific context.

The primary rationale is the complete absence of ionizing radiation. With a radiation level of 0 mSv, ultrasound poses no known risk to the developing fetus. This makes it the unequivocal first-line modality for any non-emergent abdominopelvic imaging during pregnancy. It also avoids the need for intravenous contrast agents, as gadolinium-based agents used in MRI are generally avoided unless benefits clearly outweigh potential risks, and iodinated contrast for CT is used judiciously.

From a diagnostic standpoint, ultrasound is highly effective for evaluating the key concerns in the differential diagnosis. It excels at:

  • Detecting hydronephrosis and hydroureter.
  • Identifying many renal stones, particularly those larger than 5 mm or those located in the kidney or at the ureterovesical junction.
  • Visualizing the bladder to assess for masses, stones, or ureteral jets (using color Doppler) to help determine if a ureter is obstructed.
  • Characterizing renal masses as simple cysts or more complex solid lesions.

Alternative imaging studies are rated lower for valid reasons. For example, `CTU without and with IV contrast` is rated Usually not appropriate. Despite being the gold standard for hematuria evaluation in non-pregnant adults, its use of significant ionizing radiation (☢☢☢☢ 10-30 mSv) is a major contraindication. Similarly, `MRI abdomen and pelvis without and with IV contrast` is also Usually not appropriate as an initial test, primarily due to concerns about gadolinium contrast. However, a non-contrast study, `MRU without IV contrast`, is rated May be appropriate and serves as a valuable second-line, problem-solving tool if ultrasound is non-diagnostic.

What’s Next After US kidneys and bladder retroperitoneal? Downstream Workflow

The results of the initial ultrasound will guide your next steps. The workflow branches based on whether the findings are negative, positive, or indeterminate.

If the study is positive for a clear cause:

  • Finding: Mild to moderate hydronephrosis, especially right-sided, without a clear obstructing cause. Next Step: This is likely physiologic. Reassure the patient and manage with clinical follow-up. Repeat imaging is only needed for worsening symptoms.
  • Finding: A clear renal stone with significant hydronephrosis or if the patient is symptomatic. Next Step: Consult Urology for management, which often begins with conservative measures (hydration, pain control) but may require intervention (e.g., stent placement) for intractable pain, infection, or worsening obstruction.

If the study is negative:

  • Finding: A technically adequate and completely normal ultrasound in an asymptomatic patient. Next Step: The most prudent course is observation. Plan for a repeat urinalysis postpartum. If microhematuria persists after delivery, the patient can then undergo a standard hematuria evaluation (often with cystoscopy and CT Urography) without fetal risk.

If the study is indeterminate or suspicious:

  • Finding: The ultrasound is inconclusive (e.g., severe hydronephrosis without a visualized stone, technically limited views) and clinical suspicion for a pathologic cause remains high. Next Step: This is the ideal scenario to consider the study rated May be appropriate: `MRU without IV contrast`. This non-radiation modality can better delineate the urinary tract, identify mid-ureteral stones missed on ultrasound, and characterize renal masses without using gadolinium.

Pitfalls to Avoid (and When to Get Help)

Navigating this clinical scenario requires careful consideration to avoid common errors. Be mindful of these pitfalls:

  • Dismissing physiologic hydronephrosis: Attributing flank pain or other symptoms to physiologic hydronephrosis without a thorough evaluation can lead to a missed diagnosis of an obstructing stone.
  • Defaulting to radiation-based imaging: Ordering a CT scan without first considering pregnancy status is a critical error. Always confirm pregnancy status in patients of childbearing age before ordering studies with ionizing radiation.
  • Incomplete ultrasound: Ensure the sonographer evaluates the entire urinary tract, including the bladder and ureteral jets. A study focused only on the kidneys may miss distal ureteral or bladder pathology.
  • Ignoring persistent symptoms: If a patient’s symptoms (e.g., severe flank pain, fever) persist or worsen despite a negative or “physiologic” ultrasound, do not delay further evaluation. This is a key trigger to escalate to a non-contrast MRU or consult a specialist.

If you encounter signs of sepsis, intractable pain, or worsening renal function, immediate consultation with both Urology and Maternal-Fetal Medicine is warranted.

Related ACR Topics and Tools

This article covers one specific scenario within the broader topic of Hematuria. For a comprehensive overview of imaging for all patient presentations, from asymptomatic microhematuria to gross hematuria with clots, please consult our parent guide. You can also use the tools below to explore adjacent scenarios, review imaging techniques, and discuss radiation dose with patients.

Frequently Asked Questions

Is any amount of hydronephrosis during pregnancy considered abnormal?

Not necessarily. A mild to moderate degree of hydronephrosis, particularly of the right kidney, is a common physiologic change starting in the second trimester. It is caused by hormonal effects and compression from the uterus. It becomes concerning if it is severe, left-sided without right-sided involvement, associated with a visible obstruction like a stone, or accompanied by clinical signs of infection or significant pain.

If the initial ultrasound is negative, is any further imaging needed during the pregnancy?

If the patient is asymptomatic and the ultrasound is completely negative, the standard approach is clinical observation with a plan to re-evaluate postpartum if the microhematuria persists. Further imaging during pregnancy, such as a non-contrast MR Urography, is typically reserved for cases where there is high clinical suspicion for a stone or other pathology despite the negative ultrasound, or if the patient develops new symptoms like flank pain or fever.

Why is a non-contrast MR Urography (MRU) preferred over a standard MRI of the abdomen and pelvis?

A non-contrast MR Urography is a specialized MRI sequence (heavily T2-weighted) designed specifically to visualize the fluid-filled urinary tract (kidneys, ureters, bladder) with high detail, making it excellent for detecting obstruction and ureteral anatomy without contrast. A standard non-contrast abdominal MRI may not include these specific sequences and may be less sensitive for subtle urologic pathology.

Can ultrasound reliably rule out all kidney stones?

Ultrasound is good but not perfect for detecting kidney stones. It is most sensitive for stones within the kidney and at the very top (ureteropelvic junction) or very bottom (ureterovesical junction) of the ureter. It can easily miss smaller stones located in the mid-ureter. However, it is excellent at detecting the secondary sign of a stone: hydronephrosis (swelling of the kidney), which indicates obstruction.

What if the patient has an allergy to ultrasound gel?

A true allergy to standard aqueous ultrasound gel is extremely rare. Most reactions are irritant contact dermatitis rather than a true allergy. If a known sensitivity exists, hypoallergenic, non-staining gels are widely available and can be requested. This should not be a barrier to performing this essential, radiation-free examination.

Reviewed by Pouyan Golshani, MD, Interventional Radiologist — May 29, 2026