Objective To obtain the fatal aortic isthmic systolic index(ISI)with prenatal ultrasound,and to establish reference ranges for normal live fatal aortic isthmic systolic index(ISI). Methods A total of 251 normal fetuses at 18?37 gestation weeks were collected and underwent Dop?pler ultrasound examination to obtain the fatal aortic isthmus waveforms. Peak systolic velocity(PSV)and systolic nadir(NS)were measured on the waveforms. ISI was derived from the ratio NS/PSV,and its relationship with gestational weeks was analyzed. Results ISI value was between 0.19 and 0.21,from 18 to 24 gestational weeks. As the gestational weeks increasing,PSV increased;however,NS decreased gradually. At about 28 weeks,a brief end?systolic deceleration wave was observed on the aortic isthmus waveforms,and this trend led to ISI towards a mean value of zero at about 30 weeks. At the end of pregnancy,ISI was about-0.33. The curve equations were established regarding the gestational age as the independent variable X,PSV,NS and ISI as the dependent variable Y respectively. Y∧=-69.116+9.542X-0.139X2,R2=0.96,P<0.001;Y∧=-99.881+10.331X-0.232X2,R2=0.99,P<0.001;Y∧=-0.671+0.086X-0.02X2,R2=0.99,P<0.001. Conclusion Prenatal ultrasound can measure the fetal aortic isthmic systolic index,which is helpful in the diagnosis of fetal cardiovascular anomalies and evaluation of fetal intrauterine status.