**7. The measurement error when estimating the HFUPR**

When estimating the HFUPR, the measurement error is made when assessing the volume of the bladder, and that SD has been estimated for the 2D technique and the sum-of-cylinders method. There are some other factors that influence the HFUPR measurement error, the magnitude of the HFUPR and the number and time points of bladder image capture. To date, no information relating to the SD for bladder volume estimation by 3D ultrasound, which is a prerequisite for the subsequent analysis of estimation accuracy, is available.

When utilising the HFUPR for fetal surveillance, it is necessary to know whether the estimated HUFPR is pathologically low, i.e. below the 5th percentile point. It is therefore necessary to answer the question: 1) What is the risk of false readings at the 5th percentile point, for example, even though the true HFUPR is at a higher percentile point? Furthermore, it is necessary to answer the question: 2) How much of an observed HFUPR change (for example, during daily controls) can be explained exclusively by measurement error? The implication of the volume measurement error was demonstrated in detail in a publication publication concerning the 2D technique and the sum-of-cylinders method. (Fagerquist et al., 2010).
