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
Best marker of gestational age in 2nd trimester is : 
Select and explain. 
A. Biparietal diameter, B. Head circumference, C. Crown rump length, D. Femur length
A: Biparietal diameter
Answer: A: Biparietal diameter 
Explanation: 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.