Confirmation of the Distribution of Measured Values from an Ultrasound Fetal Heart Rate Monitor

To estimate the measurement error in an ultrasound fetal heart rate (FHR) monitor, we compared the differences in the distribution of the FHR measured with an ultrasound Doppler to the FHR measured with a direct fetal electrocardiogram. We confirm that (1) the distribution is very similar to Student's t distribution, and (2) the number of degrees of freedom in Student's t distribution has a significant impact on the evaluation of fit.