Bilateral kidney ultrasonography is the most appropriate diagnostic study for this pregnant patient with suspected nephrolithiasis. Although noncontrast helical CT has traditionally been the most commonly used imaging technique for suspected nephrolithiasis because it detects most stones, provides helpful anatomic information, visualizes the entire urinary tract, and may potentially provide alternative diagnoses if nephrolithiasis is not detected, it is associated with significant radiation exposure and is therefore contraindicated in pregnant women. Kidney ultrasonography is increasingly being used as an initial diagnostic study for nonpregnant patients with suspected nephrolithiasis, particularly younger patients, to avoid significant radiation exposure, and it is the study of choice for pregnant women with possible kidney stones. Although it is less sensitive than CT for kidney stones, particularly for small stones or those in the distal urinary tract, a positive study for nephrolithiasis can exclude complications such as hydronephrosis and remove the need for more extensive additional testing.

Abdominal MRI can be used during pregnancy, but it is not optimal for imaging kidney stones. Ultrasonography is preferred, although MRI may be a diagnostic option if additional imaging is required for diagnosis.

Low-dose CT is both sensitive and specific for detecting kidney stones but is also associated with significant radiation exposure, similar to conventional stone protocol CT. Low-dose CT is absolutely contraindicated in the first trimester of pregnancy and is used only in specific situations in pregnant women in the second or third trimesters. It is not an appropriate study for this patient.

Plain abdominal radiography has limited utility for suspected nephrolithiasis due to its inability to detect radiolucent stones and the limited anatomic information it provides. It also involves radiation exposure and is not an appropriate next test in this patient.

Transvaginal ultrasonography may be used to detect distal ureteral stones in pregnant women with suspected nephrolithiasis and an unrevealing kidney ultrasound. However, it is not an appropriate next test for this patient.