This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Gynaecology & Obstetrics
[{'content': 'Gestational sac diameter of ______ is suggestive of anembryonic pregnancy, if embryo is not visible \nSelect and explain. \nA. 5 mm, B. 10 mm, C. 15 mm, D. 20 mm', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Embryo should be visible transvaginally once the mean sac diameter has reached 20 mm, otherwise the gestation is anembryonic.', 'role': 'assistant'}]