Flow velocity profile of the fetal aortic isthmus through normal gestation.

Experimental and clinical evidence have demonstrated that an increase in resistance to placental blood flow causes changes in aortic isthmic diastolic flow profile before any significant modification is observed in umbilical artery Doppler waveforms. To identify these abnormal flow profiles, the objective of this study was to document the normal flow profile in the aortic isthmus throughout pregnancy. The study included 81 normal fetuses from 17 to 39 weeks of gestation. On the isthmic flow profile, an index, named the balance index, was calculated: (peak systolic--end-diastolic velocities/forward--reverse flow velocity integrals). Before 20 weeks of gestation, a forward flow was recorded throughout the cardiac cycle and the diastolic deceleration phase was gradual and smooth. After 20 weeks, an incisura appeared at end-systole that progressively increased, and by 30 weeks of gestation a brief reverse diastolic flow was constantly recorded. The balance index increased slightly throughout gestation. Color flow mapping demonstrated that the reverse flow observed late in gestation in the isthmus was coming from the ductus arteriosus. In conclusion, the morphology of the Doppler flow velocity waveform of the fetal aortic isthmus changes with gestation. The proposed balance index may be useful in identifying fetuses with disturbed peripheral hemodynamics.