Etiology of third-trimester maternal hyperuricemia in nonpreeclamptic twin gestations

Objective: To determine whether the higher maternal serum uric acid levels observed in the third trimester of nonpreeclamptic twin gestations result from increased uric acid production or decreased renal excretion. Methods: Thirty-four nonpreeclamptic subjects with twin gestations were analyzed, along with 34 singleton controls matched for age, ethnicity, prepregnancy weight, height, and gestational age. For each subject, a serum sample and 24-hour urine specimen were obtained in the third trimester. Serum and urine uric acid and creatinine levels were determined, as well as total 24-hour urine uric acid, uric acid clearance, creatinine clearance, fractional uric acid clearance, and net tubular uric acid absorption. Results: The twin gestation group had significantly higher maternal serum uric acid levels (5.2 ± 1.2 compared with 4.0 ± 1.0 mg/dL, P < .001) and maternal serum creatinine levels (0.7 ± 0.2 compared with 0.5 ± 0.1 mg/dL, P < .001) than the paired singleton group. This was associated with greater 24-hour urine uric acid excretion (688.7 ± 167.0 compared with 597.7 ± 164.2 mg, P = .04) and 24-hour urine creatinine excretion (1268.4 ± 249.9 compared with 1161.2 ± 277.1 mg, P = .03) in the twin group. No differences were seen between the groups in uric acid clearance, creatinine clearance, fractional uric acid clearance, filtered uric acid load, or net uric acid absorption. Conclusion: The higher maternal serum uric acid levels observed in the third trimester of nonpreeclamptic twin gestations result in part from increased uric acid production, as reflected in the increased daily uric acid excretion.