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': 'At what period, Frequency is present in pregnancy: \nSelect and explain. \nA. Before 16 weeks, B. After 18 weeks, C. After 20 weeks, D. After 24 weeks', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Before 16 weeks', 'role': 'assistant'}]