Ultrasonographic evaluation of human fetal urinary flow rate: accuracy limits of bladder volume estimations

This study was conducted to examine the reliability of fetal bladder volume predictions on the commonly used ovoid volume formula and to develop a simpler but equally accurate method requiring fewer measurements.

Nine hundred twenty seven measurements were obtained from 11 dead fetuses in a water bath. Known incremental volumes of saline solution were infused and the largest linear bladder dimensions length, depth, and width, were measured ultrasonographically. Bladder volumes were calculated with the ovoid volume formula, 4/3.pi.(Length.Depth.Width)/8, and compared with true volumes. The areas of the ultrasonographic planes observed during the measurement process were calculated and plotted against the true volume for regression analysis. The sagittal and coronal areas were converted into volumes with the regression equations 0.46323 + 1.39394. Sagittal area and 1.20640 + 1.07603. Coronal area, respectively.

The fetal bladder volumes from the ovoid formula had an average error of 6.4 ml, with a 95% confidence interval of +/- 14.1 ml. The fetal bladder volumes derived from sagittal and coronal area formulas had mean absolute errors of approximately 0 ml, with 95% confidence intervals of +/- 4.0 ml and +/- 4.4 ml, respectively. The best predictions were obtained with the sagittal area measurements.

The ovoid volume formula overpredicts fetal bladder volume and has a wide 95% confidence interval. This inaccuracy probably affects measurements of fetal urinary flow rates. Use of the sagittal or coronal area affords improved accuracy and is easier and more convenient.