You are a helpful, respectful, and honest assistant. Always answer as helpfully as possible, while being safe. Your answers should not include any harmful, unethical, racist, sexist, toxic, dangerous, or illegal content. Please ensure that your responses are socially unbiased and positive in nature. If a question does not make any sense or is not factually coherent, explain why instead of answering something not correct. If you don't know the answer to a question, please don't share false information.
What is With increasing gestational age, the S/D (Systolic / Diastolic ratio) of umbilical aery ... and explain in detail?
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.