Fetal urine production and accuracy when estimating fetal urinary bladder volume.

OBJECTIVES: The volume of the fetal urinary bladder can be estimated by measuring bladder diameters on an ultrasound image. The calculated urine production rate might reflect the fetal status in compromised pregnancies. The aim of this study was to assess the fetal urine production rate and the accuracy of measurements of diameters and volumes of the bladder. DESIGN: Urine production rate and the variability in volume measurement error were assessed by ultrasound examinations of fetal bladder images documented on videotapes. In material comprising 101 longitudinal and 90 transverse bladder images, the variability (SD) in distance measurement error was estimated for the bladder diameters. Using this SD, the variability in volume measurement error in the selected bladder image was also calculated. RESULTS: The urine production rate for fetuses of 20, 25, 30, 35 and 40 weeks is 4.2, 12.1, 22.7, 36.1 and 52.2 mL/h, respectively. The SD for volume measurement error when selecting bladder images with a volume of 5, 10, 20, 30 and 40 mL is 17.3%, 13.6%, 11.8%, 11.2% and 10.9% of the actual volume, respectively. The corresponding SD when measuring the volume of selected bladder images accounted for 3.7-2.0%. CONCLUSIONS: The urine production rate presented here is in parity with recent previous reports. We distinguish between different kinds of measurement error. The variability was mostly related to the selection of appropriate bladder image, whereas measurement of the selected bladder image accounted for only a minor part of the resulting variability.