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': 'WHO definition of aboion is \nSelect and explain. \nA. Pregnancy termination or loss before 12 weeks of gestation, B. Pregnancy termination or loss before 20 weeks of gestation, C. Pregnancy termination or loss before 22 weeks of gestation, D. Pregnancy termination or loss before 24 weeks of gestation', 'role': 'user'}, {'content': "Answer: B \nExplanation: The National Center for Health Statistics, the Centers for Disease Control and Prevention, and the World Health Organization all define aboion as pregnancy termination or loss before 20 weeks' gestation or with a fetus delivered weighing <500 g.Reference: William's Obstetrics; 25th edition; Chapter 18; Aboion", 'role': 'assistant'}]