The first stage of labour lasts twelve to twenty-four hours with a first baby and usually less with subsequent children. During these hours the contractions of the uterus increase in strength and come more frequently until by the end of the first stage they may be occurring every three to five minutes. The cervix or neck of the womb undergoes several important changes. First it is drawn up so that it no longer projects into the vagina; then it gradually opens or dilates. By the end of the first stage the opening in the cervix is large enough to allow the baby’s head to pass into the vagina. The cervix itself is now almost non-existent and the uterus and vagina have been converted into one continuous cavity. In the majority of labours the membranes rupture and the waters escape towards the end of the first stage, though they may rupture much earlier, and in a few cases they do not rupture before the birth of the baby. The baby may then be born completely enclosed in the membranes or “caul,” an event which was once thought to be an omen of great good fortune.