This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Obstetrics, Subject name: Gynaecology & Obstetrics
[{'content': 'Single best parameter to assess gestational age in first trimester? \nSelect and explain. \nA. CRL (Crown Rump Length), B. BPD (Biparietal diameter), C. HC (Head Circumference), D. FL (Femur Length)', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Best parameter to assess gestational age- First trimester - CRL (Crown Rump Length) Second trimester - BPD (Biparietal Diameter) Third trimester - BPD (Biparietal Diameter) Best Overall - CRL (Crown Rump Length)', 'role': 'assistant'}]