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Figure 1 (a) The normal fetal pulmonary vein Doppler spectra is composed of forward flow in ventricular systole (S) and diastole (D), with cessation of flow during atrial systole. (b) Reverse flow during atrial contraction in PV spectra is commonly shown in fetal left heart obstruction. Measurement of velocity-time integral for forward (VTIF) and reverse (VTIR) flow is demonstrated.