<prompt> If the patient conceives in lactational amenorrhea, to determine the EDD, Dating scan uses ?Best indicator of gestational age in first trimeseter is -Gestational sacC.R.LYolk sacB.P.D  <response>: Ans. is b i.e., C.R.L GA in the first trimester is usually calculated from the fetal crown-rump length CRL. This is the longest demonstrable length of the embryo or fetus, excluding the limbs and the yolk sac. The GA estimate has a 95 confidence interval of plus or minus 6 days, and it is most accurate between 7 and 10 weeks amenorrhea. Fetal biometry in the second trimester can yield acceptably accurate estimates of GA from 12 to approximately 22 weeks of amenorrhea. The best parameters are the biparietal diameter BPD and the head circumference HC. 