What is the appropriate follow‑up for a 21‑year‑old woman at 19 weeks gestation with a urinary tract infection and MRI‑demonstrated hydronephrosis without an obstructing stone, currently on day 3 of a pregnancy‑safe antibiotic regimen?

Follow-Up Management for Pregnant Patient with UTI and Hydronephrosis

Complete the antibiotic course and perform follow-up ultrasound with color Doppler to assess resolution of infection and distinguish physiologic from pathologic hydronephrosis, using renal resistive index (RI) measurements and ureteral jet evaluation. 1, 2

Immediate Management (Days 3-7 of Treatment)

• Continue the current pregnancy-safe antibiotic regimen to complete a full course (typically 3-7 days for cystitis, longer if pyelonephritis is suspected). 3
• Obtain a repeat urine culture 7 days after completing antibiotic therapy to confirm microbiologic cure, as treatment failure occurs in up to 20% of cases. 4, 3
• Monitor for symptoms of progression to pyelonephritis including fever, flank pain, nausea/vomiting, or systemic symptoms, which would require immediate escalation of care. 2, 3

Diagnostic Follow-Up Imaging

• Perform color Doppler ultrasound of the kidneys and retroperitoneum as the definitive follow-up study after completing antibiotic therapy to reassess the hydronephrosis. 1, 2

Key Ultrasound Parameters to Assess:

• Measure renal resistive index (RI): An RI > 0.70 indicates underlying renal dysfunction requiring intervention. 1, 2
• Calculate inter-renal RI difference: A disparity ≥ 0.04 between kidneys is consistent with pathologic obstruction rather than physiologic pregnancy-related hydronephrosis. 1, 2
• Evaluate ureteral jet flow: This helps differentiate obstructive from non-obstructive hydronephrosis. 1, 2
• Measure anteroposterior diameter of the renal pelvis: Diameters > 21 mm on the right or > 25 mm on the left predict need for intervention with high sensitivity and specificity. 5, 2

Clinical Context and Pitfalls

• Physiologic hydronephrosis occurs in 70-90% of pregnant women (typically more pronounced on the right side due to dextrorotation of the uterus and progesterone-induced smooth muscle relaxation), making it critical to distinguish this benign finding from pathologic obstruction. 1, 2
• The absence of an obstructing stone on MRI does not exclude pathologic obstruction—compression from the gravid uterus, blood clots, or functional obstruction can all cause symptomatic hydronephrosis requiring intervention. 2
• Untreated symptomatic hydronephrosis may lead to preterm labor, low birth weight, or maternal/fetal death, making close follow-up essential even after UTI treatment. 2

Escalation Criteria

If ultrasound findings suggest pathologic obstruction:

• Consider MRU without IV contrast if ultrasound is equivocal or non-diagnostic, as this is preferred over CT in pregnancy and provides detailed anatomic information. 1, 2
• Indications for MRU include: anteroposterior diameter > 2 cm, persistent symptoms despite conservative management, or need for detailed anatomic mapping. 1

If intervention is required:

• Ureteral stent placement or percutaneous nephrostomy (PCN) may be necessary for symptomatic relief if conservative management fails, with catheters typically left in place until after delivery. 2
• Definitive stone intervention should be deferred until postpartum when possible. 2

Ongoing Surveillance

• Repeat urine cultures monthly for the remainder of pregnancy to screen for recurrent bacteriuria, as pregnant women with a history of UTI are at increased risk for recurrence. 3, 6
• Serial ultrasound monitoring may be warranted if initial follow-up shows borderline findings or if symptoms recur, particularly in the third trimester when physiologic hydronephrosis typically worsens. 2

Critical Pitfalls to Avoid

• Do not assume all hydronephrosis in pregnancy is physiologic—symptomatic hydronephrosis affects 0.2-4.7% of pregnant women and requires careful evaluation. 2
• Avoid CT imaging unless both ultrasound and MRI fail to establish a diagnosis, and if CT is unavoidable, use low-dose protocols limiting fetal exposure to < 20 mGy. 1, 2
• Do not use plain radiography (KUB) or intravenous urography (IVU)—these add unnecessary radiation without diagnostic benefit. 1
• Ensure antibiotic selection is pregnancy-safe throughout all trimesters—avoid sulfonamides and nitrofurantoin in the first trimester when other options are available due to potential teratogenic risks. 7