Background: Doppler study of umbilical artery, middle cerebral artery, uterine arteries and ductus venosus which is done routinely has its own limitations. The emergence of aortic isthmus index as an ideal parameter that provide an early detection of hemodynamic disturbance inside the fetus in the late pregnancy due to its position as a shunt between right and left ventricle, has inspired the study. Aims and Objectives: To determine the gestational age specific changes in fetal aortic isthmus flow indices in third trimester gestation and to compare the changes with varying time period of third trimester of gestation. Materials and Methods: A total of 124 normal pregnancies with gestational age of 27 to 40 weeks by LMP were included in the study. Study was done during July, 2018 to March, 2019 in BLK super speciality hospital, Pusa Road, New Delhi. Aortic isthmus doppler flow indices namely PSV, EDV, RI, PI, S/D, MD and TAMAX were automatically calculated in GE volusonE8 machine and GE P6 machine with 2-5 MHz sectoral array and compared by their LMP derived gestational ages. The correlations of doppler indices with gestational age was determined by linear regression equation. Results: It was observed that PSV and TAMAX increases with gestational age with correlation coefficient r=0.436 and 0.414 respectively and p value <.005. MD and ED also show a positive increasing trend with gestational age. Conclusions: This study provides normative values of PSV, EDV, PI and TAMAX with gestational age in third trimester of gestation. Studies are needed for prognostic value of these indices.