==Organs Involved==
The primary organ involved is the [[uterus]]. This contains a layer known as the myometrium which contains bundles of non-striated (smooth) muscle as well as connective tissue (collagen), nerves and blood vessels. The muscles of the inner wall of the myometrium are circular, whereas the muscles on the outer wall are longitudinal. During pregnancy this layer increases in mass approximately 20 fold, with hypertrophy increasing from 50um 50 um to 500um500 um, hyperplasia increasing and glycogen deposits increasing.
The muscle cells act as a syncytium, in which spontaneous depolarising pacemaker potentials occur. If these are above a certain threshold (known as the action potential) then a wave of calcium is released and a contraction occurs.
===Stages===
'''Stage One''' is associated with the start of painful contractions and the dilating of the cervix. Contractions are initially slow until the cervix reaches a diametre of 3cm3 cm, then the increase in speed until the end of stage one, at which point the cervix is at a diametre of approximately 10cm10 cm. Each muscle cell does not completely relax, and therefore shortens with each contraction. The retraction ring gradually moves upwards.
'''Stage Two''' is associated with the delivery of the foetus. As the foetus moves down it is rotated due to the dimensions of the pelvis (ellipsoid laterally at the pelvic inlet, circular in the middle and ellipsoid medially at the pelvic outlet). This generally results in the baby's head facing the mother's rectum. Abdominal contractions assist with the expulsion of the child. A breach birth occurs if the baby is delivered feet first.