CHAPTER VIII

THE MECHANISM OF NORMAL LABOR

The powers of labor are primarily the uterine contractions strongly aided by the muscles of the abdomen and diaphragm. Some assistance is given by the fixation of the legs and arms and sometimes by gravity, when a sitting or standing position is maintained.

The resistances are the bony pelvis and its relatively soft coverings of muscle and fascia.

The problem is to get the awkwardly shaped passenger through the curiously shaped passage.

In the first, and a part of the second stage, the uterine contractions do not act directly upon the body of the child, for the latter is surrounded by a wall of liquor amnii.

Pressure is transmitted by a fluid medium in all directions, hence, the weak part of the wall, which is the cervix, must give way. While the membranes remain intact, or when sufficient fluid is retained, no amount of pressure can injure the fœtus. When the membranes rupture, the force of the pains is exerted directly upon the child to drive it forward, and prolonged pressure may produce injurious effects through compression of fœtus, placenta, or cord.

The progress of labor is registered usually by watching the advance of the fœtal head.

The relation of the head to the pelvic brim is of great importance, as it travels much faster and easier in certain positions than in others. The term “presentation” is used to designate that part of the child which enters or tends to enter the pelvic inlet.

The presentation is named from the part of the child which comes into apposition with the brim. Thus, one speaks of a vertex presentation, or a breech presentation, or a shoulder presentation. The presentation is determined externally by palpation.

The vertex presents in 96 per cent of all labors. With the vertex presenting, the head may occupy any one of four positions. The term “position” is used to explain the relation which the most distinctive feature of the presenting part bears to the quadrants of the pelvic inlet. Thus, the most distinctive feature or landmark of the vertex is the occiput, which is the point of direction, and so again, the position is the relation of the point of direction to the brim of the pelvis. The _point of direction_ is the part that takes precedence in the process of delivery. Thus, in all cases where the occiput is in advance, the occiput is the point of direction and the position is called occipital. Where the chin is in advance, it is mental (_mentum_ is Latin for chin.) In breech cases, the sacrum is the point of direction.

The pelvis is divided by the transverse and anteroposterior diameters into four quadrants named respectively the left anterior, the right anterior, and the right and left posterior. (See Fig. 1.) Thus, in a vertex presentation the back of the child may be (and in 53 per cent is) to the front and to the left.

The occiput is the point of direction, and lies in relation to the left anterior quadrant of the pelvis, and is spoken of as a left-occipito-anterior position. Similarly a right-occipito-anterior position is named, and right- and left-occipito-posterior positions. These occur respectively in about 21 per cent, 14 per cent and 11 per cent of the cases. (Eden.)

In passing the pelvis, the fœtus not only follows the curved line of the pelvic axis, but it describes a certain series of movements which alter its relations to the pelvis.

There are five of these movements: _flexion, descent, internal anterior rotation, extension_, and _external restitution_.

=Flexion.=—Flexion is usually present before labor begins. That is, the head is bent down until the chin touches the breast. This may be modified by various conditions, but so far as it becomes extended, the mechanism is disturbed and the labor complicated, since large and less favorable diameters are brought to delivery.

Flexion is increased by pressure against the pelvic brim as labor begins.

=Descent.=—As the driving force of the contractions becomes effective, the head passes the inlet and descends to the pelvic floor. When the large diameters of the head (biparietal) have passed the inlet, the head is said to be engaged.

=Internal Rotation.=—The head most frequently enters the brim with the occiput to the left and anterior (obliquely) because it finds more room and an easier passage; but upon passing this strait and entering the roomy, true pelvis, the head must rotate so that the long diameter of the head will conform to the long diameter of the pelvic outlet, which lies in a direction just opposite to the long diameter of the inlet or brim; hence, the occiput turns forward under the pubic arch. This movement is due largely to the sloping pelvic floor and the necessity of accommodation between the head and pelvis as the child is driven forward.

Rotation is much retarded or entirely stopped when the head is extended instead of flexed or when it enters the inlet with the occiput posterior instead of anterior.

=Extension.=—After internal, anterior rotation, the head emerges at the vulva, the occiput coming out first, then in succession the vertex, forehead and face and chin. As the chin rolls out over the perineum, it moves away from the chest wall—it becomes extended.

=External Restitution.=—While the head is passing through the outlet, the shoulders are entering the pelvic inlet, and so soon as the head is released from the restraint of the vagina, it naturally falls into its normal relation to the fœtal back; hence in the position now discussed, it turns toward the left.

Therefore, we may summarize the mechanism in a normal left-occipito-anterior position of the head by saying: The head is flexed and forced into the pelvis. It descends to the pelvic floor. The occiput rotates to the front of the pelvis and impinges against the symphysis. Extension ensues in consequence of the necessity for an accommodation between the pelvis and the advancing head, and during this extension, the head delivers over the perineum. External restitution follows.

=The Effect of Labor on the Fœtal Head.=—As the head passes through the canal, it is _moulded_ by contact with the resistances. The degree of moulding is proportionate to the pressure required to drive it through. Thus, in a large head, or a relatively small pelvis, the moulding may be extreme, and changes in the scalp are common.

_Caput Succedaneum_.—Since all parts of the scalp are in contact with a resistant wall, except in the center of the birth canal, an effusion of serum takes place here, which is due to the obstruction of the venous circulation.

Swelling occurs in the subcutaneous cellular tissue, and a tumor forms—the caput succedaneum—which spontaneously disappears in twenty-four or forty-eight hours. It is useful in confirming the diagnosis of the position.

=Cephalhæmatoma.=—Following labor a tumor is sometimes found upon the head, which is often confused with a caput succedaneum.

This tumor is caused by an effusion of blood beneath the periosteum or the covering of the bone—usually a parietal bone. It is sometimes single and sometimes double, and it varies in size from a filbert to a peach. The swelling never extends across a suture. The effusion takes place gradually, and may not appear for a day or so after birth. The cause is unknown, for it occurs after normal and easy, as well as after difficult, deliveries, and after breech, as well as vertex, cases.

At first it fluctuates, then becomes hard, and in a few weeks or months is gradually absorbed. If symptoms of cerebral pressure develop, it must be remembered that hæmatoma may occur inside as well as outside the cranium.

No treatment is necessary. Puncture is inadvisable. In extremely rare instances the tumor may suppurate and require incision.