This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Obstetrics And Gynaecology, Subject name: Radiology
Most accurate estimation of gestational age on USG is done by? 
Select and explain. 
A. Head circumference, B. Abdominal circumference, C. Biparietal diameter, D. Crown rump length
D: Crown rump length
Answer: D: Crown rump length 
Explanation: Ans. is 'd' i.e., Crown rump length Determination of gestational age and assessment of fetal size* Fetal body measurements reflect the gestational age of the fetus. The following measurements are usually made: -i) Crown-rump length (CRL): - This is the most accurate method. It is to be used in first trimester. It gives very accurate estimation of the gestational age.ii) Femur length (FL): - Used in 2nd & 3rd trimester.iii) Biparietal diameter (BPD): - Used in 2nd & 3rd trimester.iv) Abdominal circumference (AC): - Used in 3rd trimester. It reflects more of fetal size and weight rather than age.v) Head circumference (HC): - Used in 3rd trimester.