Pregnancy does not change stone incidence, but physiologic hydronephrosis, altered urinary chemistry, and the constraints on imaging and treatment make management distinctive. Most stones pass spontaneously, and intervention is coordinated with obstetrics.
Physiologic Changes
- Increased renal blood flow raises GFR by 30–50% (serum creatinine and BUN run ~25% lower) and increases the filtered load of sodium, calcium, and uric acid — producing lithogenic hypercalciuria and hyperuricosuria. Placental 1,25(OH)₂D₃ further enhances hypercalciuria and suppresses PTH. Filtered inhibitors (citrate, magnesium) and urine output also rise, so a metabolic work-up is deferred until after delivery.
- Hydronephrosis of pregnancy — from progesterone-mediated ureteric smooth-muscle relaxation and, chiefly, compression by the gravid uterus; the right side dilates more, it resolves 4–6 weeks postpartum, and it is absent when the ureter does not cross the pelvic brim (ileal conduit, renal ectopia).
- Composition — in pregnancy ~74% of stones are calcium phosphate and 26% calcium oxalate, the reverse of the non-pregnant pattern.
Renal colic in pregnancy raises the risk of preterm delivery and premature rupture of membranes.
Presentation and Imaging
Flank pain (usually with haematuria) is the commonest presentation, though the gravid state can mask signs.
- Ultrasound is first-line (transvaginal US images the distal ureter).
- MRI is second-line when US is non-diagnostic — no radiation, but stones appear only as filling defects and small ones are hard to see.
- Low-dose CT (<0.19 rad) or a limited IVP (0.1–0.2 rad/film) may be used; conventional CT is avoided. Total fetal exposure should stay below the ACOG maximum of 5 rads.
Management
- Observation is first-line — 50–80% of stones pass spontaneously; follow closely given the maternal/fetal risk.
- Intervention (coordinated with obstetrics): MET (off-label; NSAIDs are contraindicated), a temporising ureteric stent or percutaneous nephrostomy (exchange every 4–6 weeks for encrustation; risks bacteriuria and stent pain), or ureteroscopy.
- Minimise fetal radiation during URS: low-dose/pulsed fluoroscopy, tight collimation, last-image-hold, a below-table x-ray source, and a lead apron beneath the pelvis. ACOG advises non-urgent surgery in the second trimester.
Self-Test
1. What are the risks of renal colic in pregnancy? Preterm delivery and premature rupture of membranes.
2. What causes physiologic hydronephrosis of pregnancy? Compression by the gravid uterus (the main factor) and progesterone-mediated ureteric smooth-muscle relaxation.
3. How can fetal radiation be reduced if ureteroscopy is needed? Place the x-ray source below the table and a lead apron beneath the patient's pelvis, with low-dose/pulsed, collimated fluoroscopy.