This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General obstetrics, Subject name: Anatomy
[{'content': 'Best marker of gestational age in 2nd trimester is : \nSelect and explain. \nA. Biparietal diameter, B. Head circumference, C. Crown rump length, D. Femur length', 'role': 'user'}, {'content': 'Answer: A \nExplanation: In the second trimester the BPD most accurately reflects the gestational age, with a variant of 7-10 days. Prediction of gestational age by USG: 8-12 weeks - Crown rump length measurement Second trimester -BPD, HC Third trimester - Femur length Ref: Williams Obs 24e pg 198.', 'role': 'assistant'}]