Symptomatic nephrolithiasis is a common nonobstetric cause of hospital admission for pregnant patients [47]. Although the rates vary within the literature, roughly 1 in 1500 pregnancies is affected by stones, but the overall rate of symptomatic stones remains the same between pregnant and nonpregnant women; 0.03–1% [48]. Presenting symptoms are similar to the general population: nausea, flank pain, urinary frequency, microscopic, and even gross hematuria are common. The vast majority of patients can be managed expectantly but cautiously; pregnant women admitted for renal colic associated with kidney stones have a higher risk of preterm delivery [49–51].

Beginning in the first trimester , gestational hydronephrosis is seen in 80% or more of pregnant women and should resolve in the postpartum period [50]. The right ureter is typically more dilated than the left, but symptomatic stone frequency is the same bilaterally [52]. This is felt to be primarily the result of compression by the dextrorotated uterus and dilation of the right ovarian vein, although serum progesterone levels may contribute to ureteric smooth muscle relaxation [53]. Hydronephrosis and resultant urinary stasis can produce a lithogenic environment.

Renal blood flow in pregnant women increases as does overall kidney size [54]. As the glomerular flow increases, the serum levels of creatinine, blood urea nitrogen, and uric acid decrease [55]. Pregnant women are at risk of stones due to this increased urinary uric acid as well as increased calcium excretion, but the risk is offset by excretion of other factors such as citrate and magnesium. Hypercalciuria is secondary to the elevation of 1, 25-dihydroxyvitamin D in the maternal bloodstream as the fetus removes calcium across the placenta; hypercalciuria is seen in all trimesters but should return to normal levels after birth [56].

Although calcium oxalate stones are the most common type of kidney stone regardless of age and gender, a greater proportion of women in childbearing years develop calcium phosphate stones than at later ages. Studies of symptomatic pregnant women suggest that calcium phosphate stones are much more common than calcium oxalate stones, at rates of 65–70% [57, 58]. The exact etiology behind this difference is still under investigation, but it has been suggested that the increased urinary calcium and slightly higher urine pH of pregnant women in the third and second trimester, in addition to increased urinary stasis, may be linked.

The medical and surgical treatment of nephrolithiasis in the pregnant patient should be coordinated between the urologist and obstetrician [59]. The rate of spontaneous passage of stones is high: from 64% to 81%, and pregnant women are often able to pass stones at an increased rate compared with nonpregnant women [52, 58]. Due to the risks of anesthesia , conservative trials for passage with opiate pain medication are often recommended as a first-line intervention [59]. Tamsulosin, an alpha-1a antagonist, is commonly prescribed as part of medical expulsive therapy; in nonpregnant patients, it has been shown to decrease pain and decrease time to stone passage [60]. It is rated as a pregnancy category B (no adequate and well-controlled studies in pregnant women) by the Food and Drug Administration; thus, its use in the pregnant population is controversial. Animal studies in rats and rabbits demonstrated no evidence of fetal harm [61]. Data in pregnant women are sparse, but Bailey et al. found that in a retrospective review of 27 women treated with Tamsulosin for symptomatic stones, women on Tamsulosin had an increased rate of spontaneous stone passage compared with controls without increased rates of obstetric or perinatal complications [62]. The majority of women in the study had symptomatic stones and subsequent Tamsulosin exposure in the second and third trimester, consistent with usual stone presentation. While later use of Tamsulosin would ideally prevent fetal exposure at the most vulnerable times of pregnancy, Tamsulosin’s use for medical expulsive therapy in the pregnant patient is off-label and must be associated with careful counseling between physician and patient, particularly before of the third trimester. The counseling must be accompanied by clear documentation in the medical record.

Imaging options for the pregnant woman with flank pain and concern for stones are often limited due to concern for fetal safety. Ultrasonography is a natural first-line choice without any concern for side effects, although long durations of Doppler signals can cause temperature elevations and should be limited during the first trimester. Unfortunately, hydronephrosis is common in pregnant women and, especially in the late term, is not specific for nephrolithiasis and can have a sensitivity as low as 34% in the detection of kidney stones [63]. The distal ureter can be evaluated with transvaginal ultrasound; this is tolerable by patients and can complete the evaluation of the entire ureter; physiological compression of the ureter should not occur below the pelvic brim in pregnancy [64]. Ureteric jets may be helpful in determining if a ureter is obstructed, although 15% of asymptomatic pregnant women may not have detectable jets [65].

Noncontrast magnetic resonance imaging (MRI) avoids radiation exposure and can help with stone identification as well. Although noncontrast computerized tomography (CT) remains the most accurate imaging modality for stone identification, MRI can detect a filling defect signifying a stone (Fig. 9.2) and also other secondary signs that suggest the presence of nonphysiological obstruction [66]. A standing column of urine within the ureter, ending at an abrupt transition point, is indicative of obstruction (Fig. 9.3) [67]. Physiological obstruction is often more tapered in shape and should not extend beyond the pelvic brim; perinephric and periureteral edema , as well as pyelonephritis, can also be seen on MRI. In the middle of the night, however, on-call radiologists may not read MRI studies, and smaller hospitals may not have MR imaging available. Gadolinium contrast increases the specificity of MRI but is not recommended in pregnant patients; it is water soluble and can cross the placenta into the fetal circulation. The risks to fetal exposure are not known, but the American College of Obstetricians and Gynecologists recommends against its use unless benefit clearly outweighs risk [68].

Noncontrast CT is an ideal method for imaging stones but exposes the fetus directly to ionizing radiation. According to the ACOG guidelines, radiation exposure through CT scans is much lower than the exposure associated with fetal harm, and if the safety of the pregnant woman is in question, CT studies should not be withheld from the patient. However, for routine imaging or equivocal cases where ultrasound or MRI is possible, CT should be avoided. The American College of Radiology guidelines support these recommendations; the risks of fetal damage by low-dose noncontrast CT are exceptionally low, particularly at doses of radiation less than 100 mGy and/or after 15 weeks of conception [69].

Stone disease which is not responsive to hydration and analgesics requires surgical intervention [59]. Cystoscopy and stent placement rapidly relieve the ureteral obstruction [37]. Likewise, those with obstruction and signs of urinary tract infection require emergent decompression via ureteral stenting. Septic patients with hemodynamic instability should undergo percutaneous nephrostomy placement to decompress the collecting system [70]. Percutaneous nephrostomy placement avoids general anesthetic and can be performed under local analgesia. Ureteral stent placement should be done using the ALARA (as low as reasonably achievable) principles. Most urological cases can be performed with ultrasound guidance rather than fluoroscopy [71–73]. In cases where fluoroscopy is required, it should be performed with minimal fluoroscopy usage and lead shielding of the fetus (Fig. 9.4).

Once the stone is treated acutely, long-term management is dependent on the risks associated with treatment and the stage of pregnancy. Ureteral stents placed to relieve the obstruction of stones typically last 3–6 months in a nonpregnant patient before requiring replacement. In the pregnant patient, ureteral stent dwell times are much shorter and may require frequent changes due to encrustation in a mere 4–6 weeks [59]. These stent changes lend themselves to increased exposure to both anesthesia and radiation. A growing body of literature suggests that ureteroscopy with holmium laser lithotripsy may be a safe and effective alternative to repetitive stent changes [59, 74, 75]. Analytical models demonstrate that stone treatment with ureteroscopy is more cost-effective than stent changes alone regardless of gestational age [76]. Ureteroscopic management should only be performed at centers capable of providing specialty services and the ancillary equipment required to handle pregnant stone patients [59].

Extracorporeal shock wave lithotripsy (ESWL) is commonplace in the treatment of renal stones but is contraindicated in the pregnant patient. Larger stones in the kidney which are not amenable to ureteroscopy should be managed expectantly until after delivery in most cases. There are newer studies which indicate that percutaneous nephrolithotomy is feasible in the pregnant patient [70, 77]. Positioning of the patient in the prone position is difficult in the later stages of pregnancy. Complex surgical intervention such as this should be left to centers of excellence familiar with the technique and proper resources for both mother and fetus.

The genitourinary system undergoes extensive changes during pregnancy. Many alterations of normal function such as infection, dilation, or obstruction of the urinary tract, and formation of renal stones can all lead to abdominal pain in the pregnant patient. Imaging strategies use ultrasonography as the main modality with the addition use of MRI when appropriate, minimizing fetal exposure to the risks of ionizing radiation. Patient management is dependent on the stage of the pregnancy and overall maternal health. Core principles include treatment of infection, drainage of the collecting system, and treatment of the offending obstruction. The majority of urological issues in the pregnant patient can be managed both safely and effectively with minimal risk to both mother and child.