Which of the following is not correct regarding Doppler studies of the umbilical circulation?

• A. Maternal smoking leads to an increase in S/D ratio
• B. Absence of end - diastolic flow is normal at term
• C. Changes in flow velocity waveforms of the umbilical artery may be important in clinical management of high - risk pregnancies
• D. Normal pregnancy shows a characteristic decrease in the Systolic/Diastolic (S/D) ratio with advancing gestational age