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Objective: To study fetal systemic arterial stiffness in normal fetuses and compromised fetuses who had umbilical placental insufficiency (UPI). Design: Prospective study. Setting: University departments. Sample: A total of 118 normal fetuses (21-40 weeks) and 55 fetuses (UPI group) with evidence of potential compromise (high umbilical artery pulsatility index). Methods: A new real-time noninvasive measurement system based on a combined Doppler ultrasound and echo-tracking system was used as a measure of aortic/systemic arterial stiffness. The augmentation index (AI) of the fetal thoracic descending aorta was measured by using simultaneous measurements of diameter pulse and flow velocity waveforms. Main Outcome Measure: Augmentation index as a measure of stiffness. Results: In normal fetuses, successful measurements for obtaining the AI were achieved in 103 of 118 fetuses. In the normal group, the AI, as well as placental resistance, decreased during the second trimester; in contrast, an increase in the AI was observed during the third trimester. Using the AI values from the normal group, the UPI group was divided into two subgroups: 29 fetuses with a normal AI and 26 fetuses with a high AI. The clinical outcome was significantly worse in the latter subgroup compared with the normal subgroup. Conclusions: The increase of afterload caused by a high umbilical placental resistance was associated with a decrease of aortic distensibility in the compromised fetuses, suggesting an alteration of aortic wall structure. [ABSTRACT FROM AUTHOR]