This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Diagnosis in Obstetrics, Subject name: Gynaecology & Obstetrics
[{'content': 'True statement regarding this fetal parameter is? \nSelect and explain. \nA. Best parameter for dating pregnancy, B. Can be measured till 10 weeks only, C. Accuracy for gestational age estimation is (+-) 7 to 10 days, D. Fetus is measured from head to toe', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. A. Best parameter for dating pregnancyCRL - Crown Rump lengtha. 1st trimester CRL is the best parameter for dating pregnancy. If carefully performed it has a variance of 3 - 5 days.b. Ideal time to perform CRL is between 7 to 10 weeks but can be done up till 14 weeks (till the embryo is 85mm long)c. The fetus is measured in the mid-sagittal plane from crown to the rump.', 'role': 'assistant'}]