Which of the following is the cut-off value of cervical length at 24 weeks of gestation for prediction of preterm delivery?

Factors required for the prediction of preterm labour includes: presence of symptoms of preterm labour, uterine contraction of >= 4 per hour, cervical length <=2.5cm, presence of fibronectin in cervicovaginal discharge between 24-34 weeks, Bishop score >=4, cervical dilation >2cm and effacement of 80%, vaginal bleeding and prior preterm bih. Ref: Textbook of Obstetrics By D.C. Dutta 6th Edition, Pages 314-315