The objective of the study was to evaluate the effect of maternal hydration status on the maternal renal collecting system during the third trimester of pregnancy. Thirty-five patients with uncomplicated singleton pregnancies were studied between 28 and 40 weeks of gestation. Ultrasound of the maternal kidneys was performed at baseline and after oral hydration with 1 L of water. Renal pelvic dilation was defined as a mean pelvic-calyceal diameter of > 10 mm. Results were analyzed with the Student paired T test and the McNemar test for comparing correlated proportions; P < 0.05 was considered significant. Prior to hydration, 19 patients (54%) had renal pelvic dilation: 7 on the right side, 2 on the left side, and 10 bilateral. Sixty minutes after hydration, 33 patients (94%) had renal pelvic dilation: 7 on the right side, 1 on the left side, and 25 bilateral. Renal pelvic dilation peaked 60 to 90 minutes after oral hydration. They concluded that hydration status substantially affects maternal renal imaging studies during the third trimester of pregnancy.