Figure 1

Doppler spectra of uterine artery flow. Pulsatility index (PI) is used as a measure of impedance of the flow of blood distal to the sampling point and is automatically calculated according to the formula \( PI=\frac{\left(s-d\right)}{mean} \) where s is the peak d is the minimum and the average is the mean maximum Doppler shift frequency over the cardiac cycle. Resistance index (RI) is automatically calculated using the formula \( RI=\frac{\left(s-d\right)}{s} \)s, peak systolic; d, end-diastolic; c, early diastolic; x, maximum diastolic frequency.