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': 'The clinical diagnosis of threatened aboion is presumed when a bloody vaginal discharge appears through a closed cervical os during: \nSelect and explain. \nA. First half of pregnancy, B. Second half of pregnancy, C. Third trimester, D. All', 'role': 'user'}, {'content': 'Answer: A \nExplanation: A;First half of pregnancy', 'role': 'assistant'}]