Imaging for Suspected Kidney Stones in Pregnant Patients

Initial Imaging Approach

The American College of Radiology recommends starting with renal ultrasound as the first-line screening examination because it is sensitive and specific for diagnosing hydronephrosis and does not expose the patient or fetus to ionizing radiation 1, 2, 3, 4
Ultrasound has nearly 100% sensitivity for detecting large stones (>5 mm) and hydronephrosis, though accuracy is poor for small stones (<3 mm) 5, 6

If ultrasound fails to establish a diagnosis and symptoms persist despite conservative management, MR urography (MRU) without contrast should be the second-line imaging modality, as it avoids fetal radiation exposure and can detect hydronephrosis and causes of renal obstruction 1, 2, 3, 4
MRU has only 69% sensitivity for identifying the site of stone impaction compared to 100% sensitivity with CT 1, 2, 3, 4

Noncontrast CT abdomen and pelvis can be used when diagnosis remains uncertain after ultrasound and MRI, but only when the clinical benefit clearly outweighs radiation risks, with most diagnostic CT studies delivering far less than 20 mGy to the uterus 1, 2, 3, 9
CT without contrast is sensitive and specific for diagnosing stones in pregnant patients, with the key principle being mitigating radiation dose by optimizing each study and reducing the total number of imaging studies to the absolute minimum necessary 1, 2, 3

The American College of Radiology recommends a practical algorithm starting with renal ultrasound, followed by MRU without contrast if ultrasound is nondiagnostic, and finally low-dose noncontrast CT if diagnosis still uncertain and clinical urgency exists 1, 2, 3, 4, 9

Physiologic hydronephrosis occurs in >80% of pregnant patients, more commonly on the right, typically beginning in the second trimester, and should not be dismissed 2
Contrast-enhanced CT protocols should be avoided as there is no documented additional benefit of IV contrast for stone detection in pregnancy, and it adds unnecessary complexity 1, 2, 3
KUB radiography has no relevant literature supporting its benefit in pregnant patients with suspected stones and should not be used 1, 2, 3, 4
MRI limitations should be recognized, as it misses approximately one-third of stone locations that CT would identify 1, 2, 3, 4