This study showed for the first time that the PWV in the descending fetal aorta can be measured with a commercially marketed ultrasound machine.
The artery’s pulse is the diameter change caused by blood pressure change, i.e. pulse pressure. PWV is widely used as an index of atherosclerosis in adults because PWV depends on the elasticity of the arterial wall. However, PWV depends not only on this elasticity but also on the pulse pressure, because PWV is influenced by the speed of change in arterial diameter which is influenced by pulse pressure. Accordingly, PWV can provide information regarding blood pulse pressure.
In clinical practice, PWV in adults is commonly determined by arterial tonometry. However, Jiang B et al.  has reported that there is no difference between PWV values measured by tonometry and Doppler ultrasound. Using dual Doppler technology, Wang Z et al.  measured PWV and demonstrated that the measurement was accurate and reproducible, not only in vitro but also in vivo. The results of the present study showed that the PWV values from the fetal descending aorta obtained with the dual Doppler method were similar to those measured with the ultrasonic phased-tracking method. These results support the idea that the dual Doppler method is accurate for measuring PWV in the fetal descending aorta.
There seemed to be a slight positive correlation between PWV and gestational weeks in both groups, but significant correlations were found in neither group. Miyashita et al.  reported a weak correlation between PWV and gestational weeks. The number of participants in the present study was small, and more study is needed before drawing conclusions.
In this study, the fetal PWV of descending aorta was increased when the mothers were given ritodrine infusion. Ritodrine is known to have cardio-vascular side effects, not only for the mother but also for the fetus. Gokay Z et al.  reported that ritodrine infusion caused an increase in the left cardiac output in human fetuses. Räsänen J reported that both the volumetric flow and the time-averaged systolic peak, mean, and end-diastolic velocities in the fetal descending aorta were increased by maternal ritodrine infusions. Though further studies are needed before drawing conclusions, the increased PWV shown in this study might indicate increased pulse pressure due to ritodrine’s βstimulant effects. The results obtained in this study are reasonable and suggest that measuring PWV with the dual Doppler technology is promising for evaluating the fetal cardio-vascular state.
One of the limitations of PWV measurements is that pulse pressure values depend not only on PWV. Here is an equation from the study by Miyashita et al. 
Pulse pressure = 2ρΔd ÷ d×PWV2
(ρis the blood density, d is the internal diameter of the artery)
Pulse pressure depends also on the elasticity of the artery and the density of the blood; however, little is known about these factors in fetuses, and we cannot yet estimate their influence. More studies are needed to clarify how best to use PWV values in the clinical settings. One possibility, however, would be to follow PWV values in compromised fetuses. This might be a useful way to avoid missing the optimal timing of pregnancy termination, since PWV is presumed to fluctuate more than other factors. Acute prolongation of PWV in a compromised fetus could indicate a menacing drop in blood pressure.