The fœtus may present at the mouth of the Womb either by the head, the most usual way—by the lower extremities, which is the most frequent way after the head—or by various parts of the trunk, which is the least frequent way of all.
In each of these three full presentations there may be certain variations, which require to be noticed. The head, for instance, may present either by the cranium or by the face; the lower part of the body may present either by the feet, the knees, or the breech, according as the legs and thighs are flexed or extended; and the trunk may present either on the right or left side, and inclined towards the back or towards the chest, though neither the back nor abdomen ever fully present.
Each of these variations may also have slight variations again. Thus the face may present full, or by one or the other cheek, and so on. These variations however, are of little practical consequence, because we only find them at the very commencement of the labor, and they always change to the full presentation.
Practically speaking therefore, there are five full presentations, viz., the Cranium, the Face, the Breech, the Feet, or knees, and the Trunk, either by the right or left side.
The varieties of these, as already remarked, not requiring any special attention from the accoucheur, providing he is not puzzled or misled by them.
POSITIONS.
The position means the particular direction in which the presenting part of the Fœtus is placed in relation to the Pelvic straits. The Pelvis itself is supposed to be divided into two similar halves, the right and the left, and each presenting part has one particular place which is referred to as the indicating point. Thus, for instance, in the Cranium the Occipit, or behind part of the head, is the indicating point; and we therefore say, in Cranium presentations, that it is a right or left Occipital position, according as the back of the head is to the right or left side of the Pelvis. In face presentations, the chin (mentor) is the indicating point, and we therefore say it is a right or left mento position, according as the chin is towards the right or left side. In breech presentations, the child's sacrum is the indicating point, and we then say it is a right or left sacral position, according as the Sacrum is towards the right or left side of the Pelvis. In trunk presentations, which are always crosswise, the head (cephalo) is the indicating point, and we therefore say it is a right or left cephalo position, according as the head lies towards the right or left side of the mother's body.
In Cranium presentations also, the back of the head is not merely on the right or left side, but may be at two different points on each side. It may be either nearest to the Sacrum (posterior), or nearest to the pubes (anterior), but still against the Ilium; it is therefore called a right or left anterior, or posterior, occipito iliac position, as the case may be. Sometimes also, the occiput lodges immediately on the pubis, instead of going to either side, and that is called an occipito pubic position; at other times, on the contrary, it is placed against the Sacrum, instead of being on either side, and that is called an occipito sacral position.
In presentations of the head therefore, we may reckon six positions—the right and left anterior and posterior occipito iliac, and the pubic and sacral.
In presentations of the face the same; they being mento iliac and so on, instead of occipito.