This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
[{'content': 'Measurement used to estimate gestational age in first trimester. \nSelect and explain. \nA. CRL, B. Femur length, C. BPD, D. Head circumference', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Crown - rump length\xa0- 1st trimeter.\nFL - 2nd and 4rd trimeter.\nAC and HC\xa0 - 3rd trimester.', 'role': 'assistant'}]