Objectives- The purpose of this study was to evaluate the effect of maternal hydration on amniotic fluid volume during maternal rest in the left lateral decubitus position. Methods- Pregnant women (n = 79) with an amniotic fluid index between 6 and 24 cm and a singleton uncomplicated pregnancy at 35 to 40 weeks’ gestation were randomized into hydration and control groups. Starting 30 minutes before the measurements, the hydration group drank 250 mL of water at 15-minute intervals (1000 mL/h). After the initial amniotic fluid index measurements, the women in both groups were instructed to rest in the left lateral decubitus position, and the measurements were repeated at 15, 30, 45, 60, 75, and 90 minutes. Results- The amniotic fluid index increased at each interval in both groups. Although each amniotic fluid index value was higher than the preceding one, only the 15- and 30-minute values in the left lateral decubitus position alone and the 15-, 30-, and 45-minute values in the left lateral decubitus position with maternal hydration were significantly higher than the preceding measurements (P < .05). A similar increase in the amniotic fluid volume was present 15 minutes after assuming the left lateral decubitus position in both groups. However, after 30 minutes, the women in the left lateral decubitus position without maternal hydration needed another 60 minutes for a significant amniotic fluid index increase, whereas the women with maternal hydration needed only another 45 minutes for a significant increase. Conclusions- Maternal rest in the left lateral decubitus position with hydration and maternal rest in the left lateral decubitus position alone caused similar increases in the estimated amniotic fluid volume at 15 minutes. However, after 30 minutes, the amniotic fluid volume increased more rapidly in the group with hydration.