labour

the process of giving birth, from dilatation of the CERVIX to expulsion of the AFTERBIRTH . It usually commences naturally, although some labours are induced (see INDUCTION ). The cervix expands, and at the same time the muscles of the UTERUS wall contract, pushing part of the AMNION down into the opening. The amnion ruptures, releasing the 'waters', but these two events do not necessarily occur at the same time. The second stage is the actual delivery of the child, who passes through the bony girdle of the PELVIS via the VAGINA to the outside. Initially the head appears at the cervix, and the uterine contractions strengthen. These contractions are augmented by abdominal muscular contractions when the baby is in the vagina. When the baby's head is clear, the whole body is helped out and the UMBILICAL CORD severed. The final stage, accomplished by some contractions, is expulsion of the PLACENTA and membranes. On average, labour lasts 12 hours (less for subsequent pregnancies), and in the second stage an EPISIOTOMY may be necessary to facilitate the emergence of the head. In most cases, the baby lies head down at delivery, although some are delivered feet or buttocks first ( BREECH PRESENTATION ).