OBJECTIVE: To estimate the effects of maternal intravenous hydration on amniotic fluid volume in normal pregnancies.METHODS: Women undergoing an amniocentesis for the evaluation of fetal lung maturity before an elective cesarean delivery were eligible to participate. An amniotic fluid index (AFI) was obtained before the amniocentesis, and at the time of the amniocentesis the amniotic fluid (AF) volume was determined by diazo-dye reaction with subsequent spectrophotometric analysis of AF samples. If the AF sample drawn for fetal maturity studies was mature, the patient was hydrated with 1000 ml, of balanced salt solution 30 minutes before her cesarean delivery. Amniotic fluid volume was subsequently estimated after the hydration by a repeat AFT. Amniotic fluid volume was directly measured at cesarean delivery and compared with the dye-determined volume. The pre- and posthydration AFT were also compared.RESULTS: A total of 17 women participated in the study between January 2001 and June 2001. Statistically significant increases in the AF volume and AFT were found. The prehydration median AF volume was 450 ml, (range 250 953), and the median increase in AF volume was 188 ml, (95% confidence interval [CI] 60, 254 mL; P < .001). Median AFI was 8.6 (range 5.8-17.8) with a median change in AFI of 1.7 cm (95% CI 1.1, 3.0; P < .001).CONCLUSION: Maternal intravenous hydration appears to increase both the actual and ultrasound-estimated AF volumes in normal third-trimester pregnancies. (C) 2003 by The American College of Obstetricians and Gynecologists.