This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Advanced Fetal Ultrasonography and Therapy, Subject name: Radiology
[{'content': 'Nuchal thickness is evaluated in: \nSelect and explain. \nA. Ante natal USG first trimester, B. Prenatal USG screening, C. Antenatal USG second trimester, D. Antenatal USG third trimester', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans. (c) Ante-natal USG second trimester* The nuchal thickness is a parameter that is measured in a second trimester scan (18-22 weeks) and it is not to be mixed with nuchal translucency (which is measured in the first trimester at 11-14 weeks).* The nuchal thickness is measured in a second trimester scan .* Nuchal translucency scan is performed between 11 and 14 weeks of gestation, because the accuracy is best in this period.* The scan is obtained with the fetus in sagittal section and a neutral position of the fetal head* The head of the fetus should neither be hyper-flexed nor extended, either of which can influence the nuchal translucency thickness.* Both are used for diagnosis of Trisomy 21 prenatally.', 'role': 'assistant'}]