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
Risk of preterm delivery is increased if cervical length is lesser than: 
Select and explain. 
A. 2.5 cm., B. 3.0 cm., C. 3.5 cm., D. 4.0 cm.
A: 2.5 cm.
Answer: A: 2.5 cm. 
Explanation: 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.