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
Cervical incompetence is when the cervix measures less than 
Select and explain. 
A. 1 cm, B. 1.5 cm, C. 2 cm, D. 2.5 cm
D: 2.5 cm
Answer: D: 2.5 cm 
Explanation: Cervical incompetence diagnosis : History of repeated midtrimester painless cervical dilatation with escape of liquor amnii followed by painless expulsion of products of conception Passage number 6-8 Hegor dilator beyond internal os without resistance Absence of opening snap on withdrawal of Hegar dilator Hysterography showing funnel shaped shadow During pregnancy ,sonography showing sho cervix <25 mm. Speculum examination showing dilatation of internal os with herniation of membranes. Ref: Dutta Obs 9e pg 161.