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As in labour with a baby in a normal head-down position, uterine contractions typically occur at regular intervals and gradually cause the cervix to become thinner and to open. In the commoner breech presentations, the baby’s bottom (rather than feet or knees) is what is first to descend through the maternal pelvis and emerge from the vagina.
At the beginning of labour, the baby is generally in an oblique position, facing either the right or left side of the mother's back. As the baby's bottom is a less efficient dilator than the head, the buttocks may remain high until late in labor. In order for the baby to pass through the mother’s pelvis, there is flexion at the side of the waist, so that one hip becomes the leading part.
The mother's pelvic floor muscles cause the baby to turn slightly so that the hips are born, usually one at a time, with the baby facing one of the mother's inner thighs. At this time the baby's shoulders are entering the mother's pelvis, and the baby's position adjusts again so that they are obliquely facing the mother's back.
The shoulders emerge as the baby’s head is entering the pelvis. The combination of maternal muscle tone and uterine contractions cause the baby’s head to flex, chin to chest. The baby's position changes again so that they are facing the mother's sacrum. Then the face emerges, and finally the back of the baby's head.
Due to the increased pressure during labour and birth, it is normal for the baby's leading hip to be bruised and genitalia to be swollen; this usually resolves shortly after birth.
Babies who assumed the frank breech position in utero may continue to hold their legs in this position for some days after birth - this is normal.