This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Miscellaneous (Gynae), Subject name: Gynaecology & Obstetrics
If the patient conceives in lactational amenorrhea, to determine the EDD, Dating scan uses ?/Best indicator of gestational age in first trimeseter is - 
Select and explain. 
A. Gestational sac, B. C.R.L, C. Yolk sac, D. B.P.D
B: C.R.L
Answer: B: C.R.L 
Explanation: 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).