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': 'Ceain sign of onset of labor: March 2013 \nSelect and explain. \nA. Labor pains, B. Show, C. Dilatation of internal os, D. Formation of bag of waters', 'role': 'user'}, {'content': "Answer: D \nExplanation: Ans. D i.e. Formation of bag of waters The first symptom to appear is intermittent painful uterine contractions followed by expulsion of blood stained mucus (show) per vaginum But formation of 'bag of membranes' is almost a ceain sign of onset of labor", 'role': 'assistant'}]