This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General obstetrics, Subject name: Gynaecology & Obstetrics
[{'content': 'Gestational sac is seen on TVS as early as : \nSelect and explain. \nA. 18 days from LMP, B. 21 days from LMP, C. 35 days from LMP, D. 42 days from LMP', 'role': 'user'}, {'content': "Answer: C \nExplanation: An intrauterine gestational sac is visualised with transvaginal sonography by 5 weeks of gestation, and an embryo with cardiac activity by 6 weeks. The embryo should be visible transvaginally once the mean sac diameter has reached 20mm.Cardiac motion is usually visible with transvaginal imaging when the embryo is 5mm. Reference: william's Textbook of obstetrics 24th edition page no 196.", 'role': 'assistant'}]