Kidney stones are solid, crystal-like masses formed from concentrated minerals and salts that develop within the kidneys or urinary tract. These formations vary in size, and their passage or obstruction causes intense pain. Urolithiasis, the condition of having urinary tract stones, is an uncommon but significant health event during gestation, occurring in approximately 1 in 1,500 to 3,000 pregnancies. This makes it the most frequent non-obstetric cause of hospitalization for acute abdominal pain in pregnant individuals. Symptoms are most often experienced during the second or third trimester.
Physiological Reasons Kidney Stones Form During Pregnancy
The body undergoes several physiological shifts during pregnancy that alter the environment of the urinary tract, increasing the risk of stone formation. Elevated levels of progesterone cause the smooth muscle lining the ureters to relax. This relaxation reduces the normal wave-like contractions (peristalsis) that push urine down the tract, leading to urinary stasis.
The growing uterus also contributes to this stasis by physically compressing the ureters, which is more commonly observed on the right side due to the uterus’s natural dextrorotation. Furthermore, the glomerular filtration rate (GFR) increases by over 50% early in pregnancy, raising the urinary excretion of stone-forming substances like calcium, uric acid, and oxalate. This altered biochemical environment, combined with the often-higher urinary pH, favors the precipitation of calcium phosphate stones.
Identifying Symptoms During Gestation
The clinical presentation of a kidney stone during pregnancy can be challenging because many symptoms overlap with common gestational complaints, such as flank pain, nausea, and vomiting. However, the pain associated with a stone is typically distinct, presenting as a severe, acute, and colicky discomfort.
This intense discomfort often begins in the flank or back and then radiates downward toward the groin or labia as the stone attempts to pass through the ureter. Hematuria, or blood in the urine, is present in up to 95% of cases, though it may only be visible microscopically. The presence of a fever or chills alongside the pain is a serious indicator of an infected urinary tract, which requires immediate medical attention.
Safe Diagnostic Approaches and Conservative Care
Diagnosing kidney stones in pregnant patients requires careful consideration to prioritize fetal safety by limiting exposure to ionizing radiation. Renal ultrasonography is the preferred initial imaging modality, as it is non-invasive and uses no radiation. Ultrasound is effective for identifying hydronephrosis (swelling of the kidney due to urine backup), and it can often visualize the stone itself.
If the ultrasound results are inconclusive, but clinical suspicion remains high, non-contrast magnetic resonance imaging (MRI) is the recommended second-line tool. Low-dose computed tomography (CT) is generally reserved for cases where other methods fail and the diagnosis is imperative, as it carries a minimal but unavoidable radiation dose. Initial management for most patients involves conservative care, which includes intravenous hydration, pain control using pregnancy-safe analgesics like acetaminophen, and monitoring for spontaneous stone passage. This expectant management is successful in helping between 46% and 84% of stones pass without further intervention.
Active Intervention and Stone Removal Options
When conservative management fails, or if complications like intractable pain, complete obstruction, or infection develop, active intervention becomes necessary. A common temporary strategy to relieve obstruction is the placement of a ureteral stent, a small, hollow tube inserted into the ureter to bypass the stone and allow urine to flow from the kidney to the bladder. Alternatively, a percutaneous nephrostomy (PCN) tube can be placed through the skin directly into the kidney to drain the urine externally.
Ureteroscopy (URS) is recognized as a definitive and safe surgical procedure for stone removal during pregnancy. A small, flexible scope is inserted through the urethra and bladder into the ureter, allowing the surgeon to visualize the stone and remove it or break it up using laser lithotripsy. URS is often preferred over serial stent exchanges because it directly addresses the cause of the obstruction and may reduce the need for multiple procedures. Procedures like Extracorporeal Shock Wave Lithotripsy (ESWL) are generally avoided because the energy transmission can pose a direct risk of harm to the fetus.
Risks to Maternal and Fetal Health
Untreated or complicated kidney stones can lead to serious risks for both the pregnant individual and the fetus. For the mother, an obstructed stone can quickly lead to pyelonephritis, a severe kidney infection. This may progress to urosepsis, a life-threatening systemic infection requiring emergency treatment. Prolonged obstruction can also result in permanent damage to kidney function.
The inflammatory stress caused by the stone and any accompanying infection increases the risk of obstetric complications. Fetal risks include the initiation of preterm labor and subsequent preterm birth, which is a leading cause of newborn morbidity and mortality. Maternal infection is also associated with an increased likelihood of low birth weight and premature rupture of membranes. Prompt diagnosis and appropriate management are essential for reducing these potential negative outcomes.