Dilatation of the urinary tract during pregnancy: Proposal of a curve of maximal caliceal diameter by gestational age

OBJECTIVE: Dilatation of the urinary tract is common during pregnancy, but the limits between physiologic and pathologic dilatation are unknown. Our purpose was to establish the pattern of physiologic urinary tract dilatation during pregnancy. STUDY DESIGN: This was a cross-sectional study. The maximal caliceal diameter was measured by ultrasonography at different weeksʹ gestational age in 1506 pregnant and 181 postpartum women. A curve with the 90th, 75th, and 50th percentiles was drawn for each kidney. RESULTS: Fifty-three (6%) pregnant and postpartum women had some degree of dilatation. The right kidney was more frequently and severely affected, as were nulliparous women. The 90th percentile of maximal caliceal diameter of the right kidney increased about 0.5 mm/wk up to 24 to 26 weeks, about 0.3 mm/wk up to 31 to 32 weeks, and remained stable until term. The left maximal caliceal diameter attained 8 mm around week 20 to 24 and remained stable until term. Only 6% had dilatation in the postpartum period. CONCLUSION: A normal curve of dilatation of the urinary tract is proposed. (Am J Obstet Gynecol 1998;178:1082-6.)