To test the hypothesis that maternal oral hydration would increase the amniotic fluid (AF) index in pregnancies with normal AF.Forty women with a normal AF index (7.0-24.0 cm) were randomized to either the control or hydration group. Women in the hydration group drank 2 L of water and returned for the post-treatment AF index in 4-6 hours, whereas women in the control group drank only 100 mL of water during the same time period. The investigator performing the AF index was blinded to the subject's group. The pre- and post-treatment AF indexes and maternal urine specific gravities were compared between the groups.The mean AF index in the hydration group increased significantly by 3.0 +/- 2.4 cm (P < or = .0001) whereas it declined significantly by 1.5 +/- 2.7 cm in the control group (P < or = .02). The maternal urine specific gravities also changed significantly in the expected direction, with those in the hydration group decreasing and those in the control group increasing (P < or = .0001). There was a regression coefficient of -0.6 (P < or = .0001) between the change in urine specific gravity and the change in AF index. The mean time between the pre- and post-treatment AF indexes was not different between the groups.Maternal oral hydration increased the AF index by approximately 16%, whereas fluid restriction decreased the AF index by 8% in women with normal AF. These findings support previous data that maternal hydration increased the AF index by 31% in women with decreased AF and suggest that maternal fluid volume or osmolality may have a role in maintaining the AF volume.