Will a history of calcium oxalate (CaOx) stones increase the risk of stone formation during pregnancy, particularly in the first, second, or third trimester?

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

The risk of stone formation for a woman with a history of calcium oxalate stones will be increased in the third trimester and immediately postpartum, peaking at 0-3 months after delivery.

Epidemiology and Risk Pattern During Pregnancy

Pregnancy creates unique physiological changes that affect kidney stone risk throughout gestation:

• First trimester: Risk is unchanged compared to non-pregnant state 1
• Second trimester: Risk begins to increase (OR 2.00) 1
• Third trimester: Risk significantly increases (OR 2.69) 1
• Immediate postpartum (0-3 months): Risk peaks (OR 3.53) 1
• Risk returns to baseline by 1 year after delivery 1

Pathophysiological Mechanisms

Several pregnancy-specific factors contribute to increased stone risk:

• Anatomical changes: Mechanical compression of ureters by the enlarging uterus (particularly after week 20) leads to urinary stasis 2
• Hormonal changes: Reduced ureteric peristalsis affects urinary flow 2
• Metabolic alterations:
  • Increased urinary calcium excretion (hypercalciuria): 251 mg/day during pregnancy vs. 121 mg/day postpartum 3
  • Higher saturation of urine for calcium oxalate (3.0 vs. 2.1 postpartum) 3
  • Higher saturation of urine for brushite (1.9 vs. 1.2 postpartum) 3

Stone Composition During Pregnancy

Interestingly, stone composition differs during pregnancy:

• 74% of stones formed during pregnancy are calcium phosphate (hydroxyapatite) 4
• Only 26% are calcium oxalate 4, 5
• This contrasts with non-pregnant women of childbearing age, where calcium oxalate stones predominate 5

This difference is attributed to pregnancy-related increases in both urinary calcium excretion and urinary pH, which favor calcium phosphate stone formation 5.

Management Recommendations

Prevention During Pregnancy

For women with history of calcium oxalate stones planning pregnancy:

• Hydration: Increase fluid intake to achieve urine volume of at least 2.5 liters daily 2
• Dietary calcium: Maintain normal intake (1,000-1,200 mg/day) 6, 2
• Sodium restriction: Limit sodium to 100 mEq (2,300 mg) daily to reduce urinary calcium excretion 6, 2
• Oxalate management: Limit intake of oxalate-rich foods if there's a history of hyperoxaluria 6, 2

Diagnostic Approach for Symptomatic Stones

If symptoms develop during pregnancy:

• First-line imaging: Renal ultrasonography (avoids radiation exposure) 6, 2
• Alternative imaging: Low-dose CT may be considered in second and third trimesters if ultrasound is inconclusive 6

Treatment Options

• Conservative management: First-line approach with hydration and appropriate pain control 2
• Interventional approaches (if conservative management fails):
  • Ureteroscopy is preferred 2
  • Ureteral stenting or nephrostomy tube placement are alternatives but require frequent exchanges due to rapid encrustation during pregnancy 2

Conclusion

Women with a history of calcium oxalate stones should be aware that their risk of stone formation will increase progressively throughout pregnancy, with the highest risk occurring in the third trimester and early postpartum period. Preventive measures should be emphasized, particularly increased fluid intake and maintaining normal dietary calcium while limiting sodium and oxalate-rich foods.