<prompt> Most accurate estimation of gestational age on USG is done by?Head circumferenceAbdominal circumferenceBiparietal diameterCrown rump length  <response>: 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. 