At the beginning of labour the child's head rests upon the brim of the pelvis; the vertex presents, with the occiput directed to one of the acetabula, the chin being bent upwards upon the chest; by which contrivance the long diameters of the head and of the pelvis correspond. When the waters have escaped, the uterus, acting forcibly upon the body of the child, propels tt, at first, downwards and backwards, in the direction of the axis of the brim. When the head has completely entered the cavity of the pelvis, a turn is effected, by which the face is thrown into the concavity of the sacrum; whilst the occiput, escaping under the arch of the pubis, protrudes at the orifice of the vagina. By this turn a double good is effected: 1st, the long diameter of the head is made to correspond with that of the outlet; and, 2dly, the shoulders are brought into a situation favorable for their passage through the brim. The pains continuing, the head is soon expelled, the symphysis pubis forming a fixed point upon which it turns round, the face in its passage sweeping over the perineum. When the shoulders have descended into the cavity, nature turns the child sideways, in order that they also may be brought into the most favorable situation for their final extrusion; they then readily pass the outlet, and are soon followed by the trunk and lower extremities.
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From Charles Waller
Elements of Practical Midwifery: Or, Companion to the Lying-in Room, 1829
