This is the presentation and position most frequently observed, perhaps fifteen out of sixteen times. In most Obstetrical works it is called the first position.
By observing the following Plates, and referring to the previous explanations of the diameters of the Pelvis, and fœtal head, in Plates IX and XXV, it will be seen that through all its changes of position, while making its exit, the head always presents by one of its shortest diameters to one of the largest diameters of the Pelvis, so that the relation between them is invariably the best that could be established; and many peculiar turnings and revolutions occur, apparently for the express purpose of bringing this about.
Before the rupture of the membranes the head presents its occipito frontal diameter, which measures four inches, to the right oblique diameter of the Pelvis, which measures four inches and a half; while its bi-parietal diameter, which measures only about three inches and a half, is presented to the other oblique diameter, also measuring four and a half—(See Plate [XIX].) Even here it will be seen that the passage is larger than the head which has to pass through it, but a more favorable position still can be obtained, by a slight movement of the Fœtus, which nature accordingly accomplishes, and also follows by others, to preserve the advantage, which will next be described.
Movements of the Fœtal Head.—There are three of these peculiar movements, each of which takes place at a particular period of the labor, and must be described separately.
First movement, flexion, and descent.—Immediately after the Membranes are broken, the contractions of the Uterus force the head into the upper strait, by the occipito frontal diameter of four inches, as already shown; but then commences the first movement, which consists in a bending of the child's chin down upon its breast, so that the forehead is carried up into the Womb; and the most prominent point of the back of the head presents to the middle of the passage, by the occipito bregmatic diameter, which is only about three inches instead of four. This of course makes the passage so much easier, and generally, immediately after this change, the head descends into the basin of the Pelvis.
It is not absolutely necessary however, for this movement to occur, for in a well formed Pelvis the head can descend without it, though not so easily; and sometimes, in fact, it does not take place, but this is unusual.
It must be recollected that this shifting of the head alters the position of the fontanelles—the posterior one being brought more to the centre of the strait, and the anterior one carried up out of reach, while previous to the movement they were both on a level. If an examination is not made therefore, till after this change, it may be difficult to determine the position, unless this is borne in mind. The following Plates show this very well.
PLATE XXVII.
It will be seen here that the neck is straightened out, and that the two fontanelles are on a line with each other.