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.
[Illustration: Fig. 44.—The child in left-occipito-anterior position.
(Lenoir and Tarnier.)]
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.
[Illustration: Fig. 45.—The child in right-occipito-anterior position.
Shows the flexion of the head intensified at the beginning of labor.
(Eden.)]
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.
[Illustration: Fig. 46 _A_.—The descent of the head in
right-occipito-anterior position. Seen from below. (Edgar.) Fig. 46
_B_.—Side view.]
[Illustration: Fig. 47.—Internal anterior rotation and extension of the
head in a left-occipito-anterior position. (American Text Book.)]
=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.
[Illustration: Fig. 48.—Extension. _A_, the chin leaves the chest; _B_,
extension in progress. (Eden.)]
[Illustration: Fig. 49.—_A_, extension completed; _B_, expulsion.
(Eden.)]
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.
[Illustration: Fig. 50.—A cephalhæmatomata. Do not confuse with caput
succedaneum. (Bumm.)]
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.