The musculature of the pregnant uterus is arranged in three strata:
An external hood-like layer which arches over the fundus and extends into the
An internal layer consisting of sphincter-like fibers around the orifices of the
tubes and the internal os.
Lying between the two, a dense network of muscle fibers perforated in all
directions by blood vessels. The main portion of the uterine wall is formed by
this middle layer which consists of an interlacing network of muscle fibers
between which extend the blood vessels. As the result of such an arrangement,
when the cells contract after delivery, they constrict the vessels and thus act a
Uterine contractions are involuntary and, for the most part, independent of extrauterine control.
It has been demonstrated that the uterus has pacemakers to produce the rhythmic coordinated
contractions of labor. These pacemaker sites are found near the uterotubal junctions, although
the pacemaker cells do not differ anatomically from the surrounding myocytes as they do in
cardiac muscle. The interval between contractions diminishes gradually from approximately ten
minutes in early labor to as little as two minutes near the end of labor. In the normal process
there is a progressive increment in the strength of contractions form approximately 20 mm of
mercury at the onset of labor to 50 to 80 mm late in labor. The effect of uterine contractions of
this frequency and intensity is twofold on the uterine cervix. First effacement consisting of
thinning of the cervix with a shortening of the endocervical canal, is produced. Secondly,
cervical dilation concurs, initially slowly as it accompanies the process of effacement of the cervix, and then more rapidly as cervical effacement has been accomplished (see Figure 1).
Progressive contractile activity of the uterus has been demonstrated throughout pregnancy. Most
of these contractions are imperceptible to the pregnant individual, but toward the end of
pregnancy they may achieve on a sporadic basis strength equivalent to those of early labor. False
labor, Braxton-Hicks contractions, and pre-labor contractions are terms that have been applied to
this uterine activity. The latter term is probably the most appropriate, and it is this uterine
activity which accomplishes a significant degree of effacement and even some dilatation in the
days or weeks prior to the onset of recognizable labor. Descent of the presenting part of the fetus
into the birth canal, particularly in a first pregnancy, is another result of pre-labor.
The definition or clinical diagnosis of labor is a retrospective one. There is no laboratory test
that gives a "labor titer" or an x-ray procedure that can define the difference between the laboring
and non-laboring patient. Realizing these limitations, the patient is diagnosed as being in labor
when a combination of conditions exist. Perhaps a good working definition may be stated as
follows: When in the presence of perceived uterine contractions, there is progressive cervical
dilation and descent of the presenting part which leads to the eventual expulsion of the products
of conception, the patient is in labor.
The "mechanism of labor" refers to the sequencing of events related to posturing and positioning
that allows the baby to find the "easiest way out." For the most part the fetus is a passive
respondent in the process of labor, while the mother provides the uterine forces and structural
configuration of the passageway through which the passenger must travel. For a normal
mechanism of labor to occur, both the fetal and maternal factors must be harmonious. An
understanding of these factors is essential for the obstetrician to appropriately intervene if the
mechanism deviates from the normal. The following definitions must be mastered to be able to
discuss and understand the mechanism of labor:
Attitude. This refers to the posturing of the joints and relation of fetal parts to
one another. The normal fetal attitude when labor begins is with all joints in
Lie. This refers to the longitudinal axis of the fetus in relation to the mother's
longitudinal axis; i.e., transverse, oblique, or longitudinal (parallel).
Presentation. This describes that part on the fetus lying over the inlet of the
pelvis or at the cervical os.
Point of Reference or Direction. This is an arbitrary point on the presenting part
used to orient it to the maternal pelvis [usually occiput, mentum (chin) or sacrum].
Position. This describes the relation of the point of reference to one of the eight
octanes of the pelvic inlet (e.g., LOT: the occiput is transverse and to the left).
Engagement. This occurs when the biparietal diameter is at or below the inlet of
the true pelvis.
Station. This references the presenting part to the level of the ischial spines
measured in plus or minus centimeters.
The single most important determinant to the mechanism of labor is probably pelvic
configuration. The classic work of Caldwell and Maloy is reviewed in the text and should be
understood. Their classification of the pelvis into four major types (gynecoid, android,
anthropoid, and platypelloid) helps the student understand the possible difficulties that may arise
in a laboring patient. A quote that should be remembered is: "No two pelves are exactly the
same, just as no two faces are the same. For each pelvis there is an optimum mechanism that
may be wholly different from the so-called normal mechanism described."
An important principle is that most pelves are not purely defined but occur in nature as mixed
types. Regardless of the shape, the baby will be delivered if size and positioning remain
compatible. The narrowest part of the fetus attempts to align itself with the narrowest pelvic
dimension (e.g., biparietal to interspinous diameters) which means the occiput generally tends to
rotate to the "most ample portion of the pelvis."
The mechanical steps the baby undergoes can be arbitrarily divided, and clinically they are
usually broken down into six or eight steps for ease of discussion. It must be understood,
however, that these are arbitrary distinctions in a natural continuum.
The following six divisions of labor are easy to use:
Flexion and Engagement. This occurs at various times before the forces of labor
Descent. This occurs as a result of active forces of labor.
Internal Rotation. This occurs as a result of impingement of the presenting part
on the bony and soft tissues of the pelvis.
Extension. This is the mechanism by which the head normally negotiates the
External Rotation(Restitution). This is the spontaneous realignment of the
head with the shoulders.
Expulsion. This is anterior and then posterior shoulders, followed by trunk and
lower extremities in rapid succession.
Abnormal mechanisms of labor do occur, and the operator must be able to recognize these early
and intervene when appropriate. The above mechanisms of labor should be come "second
nature" to the practitioner and indeed does become such by careful observation. Those patients
who have undeliverable or uncorrectable problems should be unhesitatingly delivered by the
abdominal route because inappropriate operative vaginal intervention may lead to damage to
both mother and fetus. Some of the undeliverable situations include persistent mentum posterior,
persistent brow presentation, some types of breech presentations, and shoulder presentation. (See Figure)
Emanuel Friedman in his elegant treatise on labor (1978) stated correctly that "the clinical
features of uterine contractions namely frequency, intensity, and duration cannot be relied upon
as measures of progression in labor nor as indices of normality. Except for cervical dilatation
and fetal decent, none of the clinical features of the parturient patient appears to be useful in
assessing labor progression." Friedman sought to select criteria that would limit normal labor
and thus be able to identify significant abnormalities of labor. These limits, admittedly arbitrary,
appear to be logical and clinically useful. The graphic representation of labor plotting descent
and dilatation against time has become known as the Friedman curve. It, or a modification of it,
is used extensively to evaluate laboring patients.
Figure 2. Graphic portrayal of the relationship between cervical dilatation and elapsed time in
labor (heavy line) and between fetal station and time (light line). Labor has been divided
functionally into a preparatory division (including latent and acceleration phases of the dilatation
curve), a dilatational division comprising only the linear phase of maximum slope of dilatation,
and a pelvic division encompassing the linear phase of maximum descent.
Functional classification of labor
Principal Clinical Features on the Functional Divisions of Labor
Etiologic factors that appear to be responsible for the development of prolonged latent phase
disorders in multiparas most often include excessive sedation administered during the course of
the latent phase and poor prelabor soft-tissue preparation. In addition, false labor and
myometrial dysfunction are found but can be diagnosed only retrospectively. (see Figure)
Secondary arrest of dilatation pattern with documented cessation of progression in the active phase
Prolonged deceleration phase pattern with deceleration phase duration greater than normal limits
Failure of descent in the deceleration phase and second stage
Arrest of descent characterized by halted advancement of fetal station in the second stage.
These four abnormalities are similar in etiology, response to treatment, and prognosis,
being readily differentiated from the normal dilatation and descent curves (broken lines).
Etiology of arrest disorders are as follows. The striking association with cephalopelvic
disproportion makes these disorders especially ominous; whenever encountered, arrest patterns
should signal the likelihood that a bony impediment exists. Other factors very often occur in
combination with each other and with disproportion as well. (see Figure 4)
Immediately after incising the uterus and fetal membranes, the operators fingers are insinuated between the symphysis pubis and the fetal head until the posterior surface is reached. The head is carefully lifted anteriorly and, as necessary, superiorly to bring it from beneath the symphysis forward through the uterine and abdominal incisions.
The progress of labor is measured by evaluating dilatation of the cervix and descent of the presenting
part as a function of time. When an abnormality is diagnosed, the cause is identified and the appropriate