Change in fetal urine production rate in growth-restricted fetuses after maternal meal ingestion

OBJECTIVE: To determine if fetal urine production is affected by maternal meal ingestion in growth-restricted fetuses. METHODS: We studied 25 normal-growth fetuses in uncomplicated pregnancies and 15 growth-restricted fetuses, all after 30 weeks' gestation. Serial fetal bladder volume measurements were obtained at 2-3 minute intervals with ultrasonography 2 hours before and 2 hours after maternal breakfast. The hourly fetal urine production rate in each maternal state was calculated from the bladder volume measurements. The amniotic fluid index (AFI) and the pulsatility index of both umbilical and fetal middle cerebral arteries were also measured. RESULTS: Two of the 15 growth-restricted fetuses were excluded from analysis, one because it was anomalous and the other because it was not small for gestational age at birth. In the normal-growth fetuses, the hourly fetal urine production rate increased significantly after maternal breakfast (mean +/- standard deviation 30.2 +/- 11.7 versus 41.1 +/- 14.6 mL/hour, P < .001). In contrast, in the growth-restricted fetuses, the rate did not change after maternal breakfast (24.6 +/- 6.2 versus 24.9 +/- 5.7 mL/hour). Although the urine production rate before breakfast did not differ between groups, 2 hours after maternal breakfast it was significantly lower in the growth-restricted fetuses than in the control group (normal-growth) (P < .001). The AFI also was significantly lower in the growth-restricted fetuses than in the control group (15.0 +/- 3.5 versus 18.6 +/- 5.0 cm, P < .04). There were no significant differences in the pulsed Doppler studies. CONCLUSION: In contrast to normal-growth fetuses, maternal meal ingestion for growth-restricted fetuses does not increase fetal urine production. Decreased fetal urine production in the maternal fed state may lead to decreased amniotic fluid volume in growth-restricted fetuses without obvious hypoxia.