Kidney stones are hard deposits of minerals and salts that form inside the kidneys and are a known complication during pregnancy. They are relatively uncommon, occurring in an estimated one in every 1,500 to 3,000 pregnancies. However, the physical and hormonal changes of gestation significantly alter the risk factors and complexity of managing the condition. A kidney stone is a crystalline aggregate that can block the flow of urine, leading to severe pain and potential complications for both the patient and the fetus.
Prevalence and Physiological Causes During Pregnancy
Kidney stone formation is influenced by the physiological changes that occur during gestation. The presence of a stone is often reported during the second and third trimesters, which correlates with the peak of these changes. Increased progesterone causes smooth muscle relaxation throughout the body, including the ureters (the tubes connecting the kidneys to the bladder). This relaxation reduces normal wave-like contractions (peristalsis) that move urine along, leading to stasis or slower flow.
Urinary stasis is compounded by the mechanical compression of the growing uterus, which impedes urine drainage. Stagnant urine provides a better environment for the crystallization of mineral components. Additionally, pregnancy leads to an increase in the filtration rate of the kidneys and elevated urinary excretion of calcium.
These factors, combined with a tendency for the urine’s pH to become more alkaline, promote the formation of specific stone types. Up to 74% of stones in pregnant individuals are composed of calcium phosphate, differing from the calcium oxalate stones typically seen in the non-pregnant population. While the risk of forming a stone is similar to that of non-pregnant women of childbearing age, the altered urinary environment makes the formation of calcium phosphate stones more likely.
Recognizing Symptoms
Identifying a kidney stone during pregnancy can be challenging because many symptoms overlap with typical pregnancy discomforts. The most telling sign is the onset of severe, intermittent pain known as renal colic. This pain is felt in the flank or side of the back and may radiate downward toward the groin or lower abdomen as the stone moves.
Frequent or painful urination (dysuria) and urgency are common, often mistaken for a standard urinary tract infection (UTI). Nausea and vomiting are also common symptoms of kidney stones, but these are often dismissed as morning sickness. The presence of blood in the urine (hematuria) is a strong indicator, though microscopic blood may be present in pregnancy without a stone.
A sudden, intense pain that comes in waves is a strong signal that a stone is attempting to pass through the narrow ureter. If flank pain is accompanied by fever and chills, this suggests pyelonephritis (a severe urinary tract infection) and requires immediate attention. Prompt differentiation of these symptoms is important, as untreated kidney stones can potentially lead to complications such as preterm labor.
Safe Diagnosis and Treatment Protocols
The primary concern when managing kidney stones in a pregnant individual is minimizing risk to the developing fetus, particularly avoiding ionizing radiation. Therefore, the initial and preferred diagnostic imaging technique is renal ultrasound. Ultrasound is safe and effective for identifying a stone and determining the presence of hydronephrosis (swelling of the kidney due to urine obstruction).
If ultrasound is inconclusive, a limited X-ray or low-dose computed tomography (CT) scan may be considered, but only when the diagnostic benefit outweighs the risk. Clinicians aim to keep radiation exposure below the 50 milligray threshold, the dose considered to pose a small risk to the fetus. Magnetic resonance urography (MRU) is another imaging option that does not involve radiation and may be used in complex cases.
The initial approach is conservative management, which is successful in the majority of cases. This involves promoting stone passage through increased fluid intake and controlling pain with pregnancy-safe medications, such as acetaminophen. Non-steroidal anti-inflammatory drugs (NSAIDs) are avoided due to risks to the fetus.
If conservative measures fail, or if the patient develops a severe infection, intractable pain, or kidney function impairment, intervention is necessary. These procedures are designed to temporarily relieve the obstruction, allowing the urine to drain and bypass the stone. The most common interventional procedures involve placing a ureteral stent (a small tube inserted into the ureter) or a percutaneous nephrostomy tube, which drains the kidney externally.
Ureteroscopy, where a small scope is inserted to visualize and remove or break up the stone, is increasingly used as a safe and definitive treatment option during pregnancy. Procedures using shock waves to break up the stone, known as extracorporeal shock wave lithotripsy (ESWL), are not performed during gestation. The decision for intervention is always made through a collaborative approach between the obstetrician and urologist, prioritizing the immediate safety of the patient and the pregnancy.