A primigravida female at term presents to the clinic with labor pains. Which of the following is a sure sign of labour?

Formation of bag of waters is almost a ceain sign of onset of labour. During dilatation of the cervical membrane the unsuppoed lower pole of the fetal membranes tends to bulge into the cervical canal. It is called bag of waters when this bulging membrane contains amniotic fluid. When uterine contractions occur this bag becomes tense and convex, and it disappears when the contraction passes off. Features of true labour pains: Occurrence of painful uterine contractions at regular intervals Contraction with increasing intensity and duration Show Progressive effacement and dilatation of cervix Formation of bag of waters Ref: Textbook of Obstetrics By D.C Dutta, 6th edn, page 117.