Acute maternal rehydration increases the urine production rate in the near-term human fetus

OBJECTIVE: We sought to investigate the effect of a decrease of maternal pl asma osmolality produced by hypotonic rehydration on the fetal urine produc tion rate in normal near-term human fetuses. STUDY DESIGN: Twenty-one healthy pregnant women attending the clinic for an tenatal care were studied between 37 and 40 weeks' gestation. The fetal uri ne production rate was assessed by serial measurements of 3 diameters of th e fetal bladder. The hourly fetal urine production rate was determined by l inear regression analysis of the calculated bladder volumes versus time and was initially determined after a period of 4 hours of fluid deprivation. T hereafter, the women were asked to drink 1 L of water, and the hourly fetal urine production rate was assessed again. The hourly fetal urine productio n rate was only studied during behavioral state 1F because it is dependent on the behavioral state. The fetal behavioral state was determined by asses sment of fetal heart rate, fetal eye movements, and fetal body movements. RESULTS: Successful recordings were obtained in 10 of the 21 women. The hou rly fetal urine production rate increased significantly after hypotonic reh ydration (P < .02). Compared with the initial hourly fetal urine production rate after 4 hours of fluid deprivation, the hourly fetal urine production rate showed an increase of 63.2% after hypotonic rehydration, from 38.2 +/ - 16.3 mL/h to 62.4 +/- 34.6 mL/h (mean +/- SD). After rehydration, the bas eline fetal heart rate fell significantly, from 141 +/- 6 to 132 +/- 8 beat s/min (mean +/- SD; P = .005). CONCLUSION: The fetal urine production rate is augmented after acute matern al oral hypotonic rehydration after 4 hours of fluid deprivation. The curre nt findings demonstrate that the near-term human fetus can handle such acut e changes in fluid osmolality by increasing the urine production rate to ma intain its fluid homeostasis. This mechanism implies that changes in matern al plasma osmolality and volume probably play an important role in determin ing amniotic fluid volume. Therefore the application of maternal hydration for the treatment of oligohydramnios should be further investigated.