What is the risk of urinary stone formation during pregnancy in a woman with a history of calcium oxalate (CaOx) stones?

Risk of Stone Formation During Pregnancy in a Woman with History of Calcium Oxalate Stones

A woman with a history of calcium oxalate stones has an increased risk of stone formation during pregnancy, particularly in the second and third trimesters, with the peak risk occurring in the immediate postpartum period (0-3 months after delivery). 1

Risk Timeline During Pregnancy

The risk of symptomatic kidney stones follows a specific pattern during pregnancy:

• First trimester: Similar risk as non-pregnant state (OR 0.92) 1
• Second trimester: Risk doubles (OR 2.00) 1
• Third trimester: Risk nearly triples (OR 2.69) 1
• 0-3 months postpartum: Peak risk period (OR 3.53) 1
• Returns to baseline by 1 year after delivery 1

Physiological Factors Contributing to Increased Risk

Several pregnancy-related changes contribute to this increased risk:

• Anatomical changes: Mechanical compression of ureters by the enlarging uterus, particularly after week 20, leading to urinary stasis 2
• Hormonal changes: Reduced ureteric peristalsis 2
• Metabolic changes: Pregnancy-related hypercalciuria and hyperuricosuria 3
• Stone composition: Unlike non-pregnant women where calcium oxalate stones predominate, stones formed during pregnancy are more commonly calcium phosphate (hydroxyapatite) (74% vs 26% calcium oxalate) 4

Clinical Implications and Management

For this patient planning pregnancy with a history of calcium oxalate stones:

1. Pre-pregnancy counseling:

   • Increase fluid intake to achieve urine volume of at least 2.5 liters daily 2
   • Maintain normal dietary calcium intake (1,000-1,200 mg/day) 2
   • Limit sodium intake to reduce urinary calcium excretion 2
   • Consider limiting oxalate-rich foods if she has history of hyperoxaluria 2

2. During pregnancy:

   • Continue high fluid intake
   • Monitor for symptoms of renal colic (sudden onset unilateral flank pain radiating to groin) 5
   • If symptoms develop, first-line management is observation with adequate hydration and appropriate pain control 2, 5

3. If stones develop during pregnancy:

   • First-line imaging: Renal ultrasonography 2, 3
   • First-line treatment: Conservative management with observation, hydration, and appropriate analgesia 2, 5
   • If conservative management fails: Ureteroscopy is the preferred interventional approach 2, 5
   • Ureteral stenting or nephrostomy tube placement are alternatives but require frequent exchanges due to rapid encrustation during pregnancy 2

Important Considerations

• Maternal and fetal risks: Stone events during pregnancy carry increased risk of maternal and fetal morbidity 2
• Premature labor: The most significant risk of urolithiasis during pregnancy is inducing premature labor 2, 6
• Medication limitations: NSAIDs (e.g., diclofenac) are contraindicated in pregnancy 5
• Coordination of care: Any interventional management should be coordinated with the obstetrician 2, 5
• Radiation exposure: Limited imaging studies should be used, with ultrasound as the primary diagnostic tool 2, 3

Follow-up Recommendations

• Close monitoring throughout pregnancy, especially in second and third trimesters
• Continued vigilance in the immediate postpartum period when risk peaks
• Definitive stone treatment, if needed, is typically deferred until after delivery 2
• Long-term preventive measures should be continued as there remains a modest increased risk of stone formation even beyond 1 year after delivery 1