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': 'Best marker of gestational age in 2nd trimester is: \nSelect and explain. \nA. Biparietal diameter, B. Head circumference, C. CRL, D. Femur length', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. is a, i.e. Biparietal diameterRef Williams Obs. 24/e, p 198; USG in Obs. and Gynae by Callen 4/e, p 208"In the second trimester the BPD most accurately reflects the gestational age, with a variation of 7 to 10 day."Ref. Williams 24/e, p 198Prediction of fetal gestational age using ultrasound biometric parametersRef Bedside Obs/Gynae by Richa Saxena, p 191Period of gestationUltrasound parameter to be used8 weeks to 12 weeksCrown-rump length measurementSecond trimesterBPD, HCThird trimesterFemur lengthNote: In brachycephaly or Dolichocephaly - HC is better parameter in 2nd trimester than BPD as HC is unaffected by the shape of fetal head.', 'role': 'assistant'}]