<prompt> With increasing gestational age, the SD Systolic Diastolic ratio of umbilical aery ...IncreasesDecreasesShows a persistent diastolic notchRemains constant  <response>: The systolicdiastolic SD ratio of flow velocities was measured as an index of peripheral resistance. The SD 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. 