How the baby's head comes out during labour

The body of uterus, cervix and vagina together form an uniformly curved canal called the birth canal. Normally, at the onset of labour with the head non-engaged, the pelvic structures anterior to the vagina are urethra and bladder and those posterior to the vagina are the pouch of Douglas with coils of intestine, rectum anal canal and perineum.
                                    As the head descends down with progressive dilatation of the vagina, it displaces the anterior structures, upwards and forwards and the posterior structures downwards and backwards, as if the head is passing through a swing door. The bladder which remains a pelvic organ throughout the first stage becomes an abdominal organ in the second stage of labour. There is no stretching of the urethra. The urethra is pushed anteriorly, with the neck of the bladder still lying in the vulnerable position behind the symphysis pubis. The changes in the posterior structures due to downward and backward displacement are marked when the head is sufficiently low down and in the stage of 'crowing'. The perineum which is a triangular area of about 4 cm thickness becomes a thinned out, membranous structure of less than 1 cm thickness. The anus, from being a closed opening, becomes dilated to the extent of 2-3 cm. The canal becomes almost a semicircle.