Acute Changes in 24-Hour Lithogenic Urine Measures Intra- and Post-Partum

To measure lithogenic changes during pregnancy in patients on a standardized diet without a prior history of nephrolithiasis.

IRB approval was obtained. The metabolic needs of pregnant participants were determined, and standardized diet administered with Boost shakes and unlimited water. 24-hour urine collections were obtained during the third trimester, post-partum, and post-lactation. Subjects remained on the controlled diet for 24-hour prior to and during the collection. Standard statistical analysis was performed.

Of 18 patients, 6 had a de novo stone event during pregnancy. Of these, 83% were composed of calcium phosphate. Stone events with calcium phosphate presented later than calcium oxalate. Stone-formers had an intra-partum body mass index greater than non-stone formers (33.1 vs 26.7, P <.05). All participants had hypercalciuria, but stone-formers were found to have a nearly 2X greater urine calcium compared to non-stone formers on their controlled diet (392.1 vs 205.9, P <.05). Post-partum, we observed a decrease in urine calcium, citrate, pH, and supersaturation of calcium oxalate and phosphate (P <.05).

This is the first study to identify lithogenic changes during pregnancy in de novo stone formers on a standardized diet. We show that stone-formers have primarily calcium phosphate stones. They also have an elevated body mass index and are significantly more hypercalciuric without other demonstratable lithogenic differences compared to non-stone formers. Post-partum, we demonstrate reduction in intra-partum lithogenicity that may mitigate stone risk. These data suggest an underlying mechanism exists in stone formers that increases urine calcium above what would be expected in gestational hypercalciuria.