This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Intra Uterine Growth Restriction, Intrapaum and Antepaum Fetal Surviellance, Subject name: Gynaecology & Obstetrics
[{'content': 'With increasing gestational age, the S/D (Systolic / Diastolic ratio) of umbilical aery ... \nSelect and explain. \nA. Increases, B. Decreases, C. Shows a persistent diastolic notch, D. Remains constant', 'role': 'user'}, {'content': 'Answer: B \nExplanation: 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.', 'role': 'assistant'}]