Total uterine volume: A new graph and its clinical applications

The prolate ellipse formula has been shown to be inaccurate and inconsistent in the calculation of total uterine volume. The stepped area-to-volume technique has proven to be both accurate and consistent and provides a value equal to the true uterine volume. When these true volumes were plotted on previously published graphs of total uterine volume derived from the prolate ellipse formula, the graphs were found to be inaccurate. New graphs based on normal values have been constructed, comparing total uterine volume with both biparietal diameter and average gestational age. When 26 abnormal values were plotted on the total uterine volume versus biparietal diameter graph, almost all fell outside the 90 per cent confidence limits. Seven of the abnormal value were from fetuses found to have growth retardation. Six of these cases, including two not appreciated either clinically or ultrasonographically, were detected by this method. The stepped area-to-volume technique should allow reconsideration of the total uterine volume concept and aid in the detection of the subtle changes in uterine size.