Which among the following is the BEST parameter for estimation of fetal age by ultrasound in third trimester?

In the third trimester femur length is most accurate to estimate the gestational age. Gestational age in the first trimester is usually calculated by from fetal crown rump length. This is the longest demonstrable length of the embryo or fetus excluding the limbs and the yolk sac. Four fetal parameters routinely measured in the second and third trimester are BPD, head circumference, abdominal circumference and femoral length. In the second trimester, the best parameters for estimating gestational age are biparietal diameter and head circumference. When BPD is altered head circumference has the advantage of being shape independent and can be used as an alternative. Femoral length can also be used and is nearly as accurate as head measurements. Ref: Diagnostic Ultrasound By L. C. Gupta, page 175, Oxford Desk Reference: Obstetrics and Gynaecology edited by Sabaratnam Arulkumaran page 48.