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
Parameters used to estimate gestational age in last trimester – a) CR lengthb) Abdominal circumferencec) BPDd) Femur length 
Select and explain. 
A. abc, B. acd, C. ac, D. bcd
D: bcd
Answer: D: bcd 
Explanation: Estimation of age by USG

1st trimester :- Crown - rump length (CRL).
2nd trimester :- Femoral length (FL), Biparietal diameter (BPD)
3rd trimester :- Femoral length, Biparietal diameter, abdominal circumference (AC), head circumference (HC).