<prompt> Parameters used to estimate gestational age in last trimester a CR lengthb Abdominal circumferencec BPDd Femur lengthabcacdacbcd  <response>: Estimation of age by USG1st trimester :- Crown - rump length CRL.2nd trimester :- Femoral length FL, Biparietal diameter BPD3rd trimester :- Femoral length, Biparietal diameter, abdominal circumference AC, head circumference HC. 