This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Preterm Labour, Subject name: Gynaecology & Obstetrics
[{'content': 'Risk of preterm delivery is increased if cervical length is lesser than: \nSelect and explain. \nA. 2.5 cm., B. 3.0 cm., C. 3.5 cm., D. 4.0 cm.', 'role': 'user'}, {'content': 'Answer: A \nExplanation: The chance of preterm labour increases more if in the second trimester the cervical length is lesser than 2.5 cms. The chances of effacing this cervix are rapid then with a smaller cervix.', 'role': 'assistant'}]