This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Diagnosis of Pregnancy, Subject name: Gynaecology & Obstetrics
[{'content': "At what size of mean gestational sac diameter measured on TVS with no embryonic pole is labelled as 'Anembryonic Gestation' - \nSelect and explain. \nA. 15mm, B. 20mm, C. 25mm, D. 30mm", 'role': 'user'}, {'content': 'Answer: C \nExplanation: An intrauterine gestational sac is reliably visualized with transvaginalsonography by 5 weeks, and an embryo with cardiac activity by 6 weeks The embryo should be visible transvaginally once the mean sac diameter hasreached 25 mm--otherwise the gestation is anembryonic. Cardiac motion isusually visible with transvaginal imaging when the embryo length reaches 5 mm.', 'role': 'assistant'}]