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The normal pattern of pulmonary vein flow in a fetus. There is a systolic peak (S) followed by an early diastolic peak (D) and flow cessation or reversal during late diastole or atrial systole (AS). The systolic and diastolic velocities are similar and there is a mild rise in velocity of forward flow with advancing gestation with a range of 10 cm/s to 40 cm/s between 16 weeks and term.
