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

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

A woman with a history of calcium oxalate stones has an increased risk of stone formation during pregnancy due to pregnancy-induced hypercalciuria, despite protective physiological mechanisms that may be present. 1, 2, 3

Physiological Changes During Pregnancy Affecting Stone Risk

Factors That Increase Risk

• Hypercalciuria: Urinary calcium excretion increases 1-2 fold during pregnancy 4
• Elevated urine pH: Pregnancy causes increased urine pH, which can promote calcium phosphate stone formation 2
• Calcium oxalate supersaturation: Pregnancy raises urine supersaturation with respect to calcium oxalate to levels as high as those seen in stone-forming women 2, 3
• Brushite supersaturation: Similarly elevated to pathological levels during pregnancy 3

Protective Mechanisms

• Increased urinary thiosulfate: Endogenous thiosulfate increases considerably during pregnancy (to approximately 36-40 μM/24 hour across trimesters), which may provide some protection 4
• Stone composition shift: During pregnancy, stones are more commonly composed of calcium phosphate (74%) rather than calcium oxalate (26%), which differs from the typical pattern in non-pregnant women of similar age 1

Stone Risk Management During Pregnancy

Dietary Recommendations

• Maintain normal calcium intake: 1,000-1,200 mg/day from dietary sources is recommended, as higher dietary calcium reduces stone risk by binding oxalate in the gut 5
• Limit sodium intake: Target <2,300 mg/day as high sodium increases urinary calcium excretion 6, 5
• Limit oxalate-rich foods: For those with calcium oxalate stones and relatively high urinary oxalate 6
  • Foods to limit include certain nuts (almonds, peanuts, cashews, walnuts, pecans), certain vegetables (beets, spinach), wheat bran, rice bran, and chocolate 6
• Increase fluid intake: Target urine output >2 liters/day to dilute stone-forming constituents 5
• Choose beneficial beverages: Coffee, tea, orange juice may be associated with lower stone risk compared to sugar-sweetened sodas 5

Monitoring Considerations

• 24-hour urine collection: Should be considered to assess stone risk parameters including volume, pH, calcium, oxalate, citrate, sodium, and potassium 5
• Timing of calcium intake: Calcium should be consumed with meals to enhance gastrointestinal binding of oxalate 6

Important Caveats

• Despite the increased risk factors during pregnancy, symptomatic stone disease is not a common complication of pregnancy 4, 2
• The physiological hypercalciuria and hyperthiosulfaturia typically return to normal values within one month after delivery 4
• If calcium supplements are needed during pregnancy, they should be taken with meals to maximize oxalate binding and minimize stone risk 6
• Women with a history of stones should be particularly vigilant about maintaining adequate hydration during pregnancy

While pregnancy creates conditions that theoretically increase stone risk, the actual incidence of symptomatic stone disease during pregnancy appears to be limited by protective mechanisms such as increased urinary thiosulfate. Nevertheless, women with a prior history of stones should be considered at higher risk and managed accordingly.