With increasing gestational age, the S/D (Systolic / Diastolic ratio) of umbilical aery ...

The systolic/diastolic (S/D) ratio of flow velocities was measured as an index of peripheral resistance. The S/D ratio normally declines from appx. 4 at 20 weeks gestation, to generally <3 after 30 weeks and finally to 2 at bih Doppler indices show a continuous reduction throughout the second half of pregnancy without any plateau or increase near term.