What is the risk of stone formation during pregnancy in a woman with a history of recurrent calcium oxalate stones?

Risk of Kidney Stone Formation During Pregnancy with History of Calcium Oxalate Stones

The risk of stone formation during pregnancy is increased for a woman with a history of recurrent calcium oxalate stones due to pregnancy-induced hypercalciuria and elevated urinary supersaturation of calcium oxalate.

Physiological Changes During Pregnancy That Increase Stone Risk

Pregnancy creates several changes that elevate stone risk:

• Hypercalciuria: Pregnant women experience significant hypercalciuria, with urinary calcium excretion reaching 251 ± 127 mg/day during pregnancy compared to 121 ± 67 mg/day post-partum (p < 0.001) 1
• Elevated supersaturation levels: Pregnancy increases urinary supersaturation with respect to calcium oxalate (3.0 ± 1.1 during pregnancy vs. 2.1 ± 1.0 post-partum, p < 0.02) 1
• Unchanged protective factors: Citrate and magnesium excretion do not increase proportionally with calcium excretion during pregnancy 2
• Increased urine pH: This further contributes to stone risk 2

Stone Composition During Pregnancy

• Calcium phosphate predominance: 74% of kidney stones detected during pregnancy are composed predominantly of calcium phosphate (hydroxyapatite), while only 26% are composed of calcium oxalate 3
• Contrast with non-pregnant state: This differs from the usual pattern in non-pregnant women of comparable age, where calcium oxalate stones are more common 3

Risk Assessment and Management

For a woman with history of calcium oxalate stones planning pregnancy:

1. Pre-pregnancy evaluation:

   • Complete stone analysis if available 4
   • 24-hour urine collection to assess baseline parameters (volume, pH, calcium, oxalate, citrate, sodium) 4

2. During pregnancy:

   • Fluid intake: Prioritize increased fluid intake targeting urine output of at least 2.5 liters daily 4
   • Dietary recommendations:
     • Maintain adequate calcium intake (1,000-1,200 mg/day) 4
     • Limit sodium intake to ≤2,300 mg/day 4
     • Reduce non-dairy animal protein intake 4
     • Avoid high-oxalate foods if urinary oxalate is elevated 4
     • Increase potassium-rich foods 4

3. Monitoring:

   • Regular assessment of urinary parameters during pregnancy, particularly calcium, oxalate, and citrate levels 4
   • Evaluation for crystalluria as a marker of therapeutic efficacy 4

Preventive Strategies

• Hydration: The cornerstone of prevention is increased fluid intake to dilute stone-forming substances 4
• Dietary modifications: As outlined above, with particular attention to maintaining adequate calcium intake rather than restricting it 4, 5
• Medical therapy considerations:
  • Standard preventive medications like thiazides may need to be reconsidered during pregnancy due to potential risks
  • Potassium citrate may be considered if hypocitraturia is present, but should be used with caution during pregnancy 5

Conclusion

While pregnancy physiologically increases the risk of stone formation through hypercalciuria and elevated urinary supersaturation of calcium oxalate, appropriate preventive measures can help mitigate this risk. The focus should be on increased fluid intake and appropriate dietary modifications, with careful consideration of medical therapy options based on individual risk factors.