This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Antenatal Care & Assessment of Fetal Well Being, Subject name: Gynaecology & Obstetrics
[{'content': 'Most reliable criteria for gestational age estimation in 1st trimester ultrasound is: \nSelect and explain. \nA. CRL, B. Femur length, C. BPD, D. Abdominal circumference', 'role': 'user'}, {'content': 'Answer: A \nExplanation: A i.e. CRLRef: Textbook of Obstetrics by Sheila Balakrishnan, 2nd edition, page 476Explanation:First trimester scan is also called Dating scan.Crown rump length is measured from the top of the head to the bottom of the buttocks excluding limbs.CRL between 9 and 11 weeks is the most accurate method of dating.Error is about +1-5 days.CRL in cm +6.5 gives a rough estimate of gestational age in weeks.Useful prior to amniocentesis and chorionic villous sampling and in diagnosis of fetal growth restriction.', 'role': 'assistant'}]