Fetal Urine Production in Late Pregnancy

Objective . Hourly fetal urine production rate (HFUPR) was studied in relation to both gestational age and the onset of spontaneous labor in normal term human pregnancies. Methods . Serial volume measurements were obtained from longitudinal ultrasound images of the fetal bladder at 1–5-minute intervals, and HFUPR was subsequently calculated. A total of 178 adequate bladder-filling cycles were recorded in 112 women, and the amniotic fluid index (AFI) was assessed. Results . HFUPR did not change significantly between 37 and 42 weeks' gestation. However, HFUPR decreased during the last 14 days prior to the onset of spontaneous labor ( P < 0.005). No significant correlation was found between HFUPR and AFI, neither when measured at the same time nor when HFUPR and AFI were measured at various intervals in time. Conclusion . HFUPR falls before and in relation to the time of onset of labor rather than in relation to gestational age.