7.1. FACTORS DETERMINING THE MECHANISM OF LABOR
The greatest effort associated with childbirth is required of the parturient woman at the end of the expulsion stage. Even in the most favorable match between the shape and size of the fetal head, and the lesser pelvis dimensions, certain conditions are necessary for the head to pass through the birth canal. In most cases the fetus overcomes all the obstacles on the part of the birth canal as the progressive advance of the fetus (and especially of its presenting part, the head) combines with other motions: flexion, rotation, extension. Besides, the head undergoes moulding.
Background information. The mechanism of labor began to be studied by Ould as late as in 1742. Ould pointed out that at the inlet of the true pelvis the fetal head is engaged in the oblique diameter and is delivered through the anteroposterior diameter.
J.J. Fried and G.A. Fried, famous professors of obstetrics from Strasbourg, published their works in 1742 and 1769 where they offered their explanation that fetal head rotation was due to the difference in dimensions of different pelvic parts and the need of the head to adapt to them.
At the beginning and in the middle of the 18th century a considerable contribution to this field of obstetrics was made by Puzos (1686-1753) and especially by Smellie (end of 17th century - 1763) who believed the relationship of pelvic dimensions and the head to be the crucial factor in understanding the mechanism of labor.
Roederer (1726-1763) divided the mechanism of labor into 5 stages, and Stein senior (1737-1803) called attention to oblique pelvic planes and invented the pelvim-eter, an instrument to measure the fetal head, to determine its length and weight before childbirth, and to determine the angle of pelvic inclination as well (see Fig. 4.10).