Fragile X syndrome is an X-linked disorder that causes mental retardation. It is caused by a repeat in the
4. Identifying the cephalic prominence. As long as the cephalic prominence is easily palpable, the vertex is not
likely to have descended to zero station.
Palpation of the uterus during a contraction may also be helpful in determining the intensity of that particular con-traction. The uterine wall is not easily indented with firm palpation during a true contraction, but may be indented during a Braxton Hicks “contraction.”
A digital vaginal examination allows the examiner to determine the consistency and degree of effacement and degree of dilation of the cervix. This examination should be avoided in women with premature rupture of membranes or vaginal bleeding. Effacement is the shortening of the cervical canal from a length of about 2 cm to a mere circular orifice with almost paper-thin edges. Effacement is expressed as a percent of thinning from a perceived uneffaced state (Fig. 8.3). A cervix that
Pregnant uterus at term, not in
labor
Active segment
Passive segment
Cervix
Upper uterine segment
Lower uterine segment
External os Pregnant uterus
early stage I labor
Pregnant uterus early stage II
labor
FIGURE 8.1. Mechanism of effacement, dilation, and labor. With continuing uterine contractions, the upper uterus (active segment) thickens, the lower uterine segment (passive segment) thins, and the cervix dilates. In this way, the fetus is moved downward, into and through the vaginal canal.
The initial examination of the patient’s abdomen may be accomplished using Leopold maneuvers, a series of four palpations of the fetus through the abdominal wall that helps accurately determine fetal lie, presentation, and position (see Figure 9.7).
Lie is the relation of the long axis of the fetus with the maternal long axis. It is longitudinal in 99% of cases, occa-sionally transverse, and rarely oblique (when the axes cross at a 45-degree angle, usually converting to transverse or longitudinal lie during labor). Presentation is determined by the “presenting part,” that is, that portion of the fetus lowest in the birth canal, palpated during the examination.
For example, in a longitudinal lie, the presenting part is either breech or cephalic. The most common cephalic presentation is the one in which the head is sharply flexed onto the fetal chest such that the occiput or vertex
pre-is not effaced, but pre-is softened, pre-is more likely to change with contractions than one that is firm, as it is earlier in preg-nancy. If the cervix is not significantly effaced, it may also be evaluated for its relative position, that is, anterior, midposi-tion, or posterior in the vagina. A cervix that is palpable ante-rior in the vagina is more likely to undergo change in labor sooner than one found in the posterior portion of the vagina.
This suggests that the presenting part has descended into the pelvis, creating more pressure on the cervix, thereby rotating it anteriorly. With more effective force on the lower uterine segment, contractions would cause a greater change in dilation and effacement of the cervix.
Fetal Station
Fetal station is determined by identifying the level of the fetal presenting part in the birth canal in relation to the ischial spines which are located approximately halfway between the pelvic inlet and the pelvic outlet (Fig. 8.4).
If the presenting part has reached the level of the ischial spines, it is termed zero station. The distance between the ischial spines to the pelvic inlet above and the distance from the spines to the pelvic outlet below are divided into fifths, and
LOP
LOT
LOA
ROP OP
Posterior fontanel
Coronal suture
Coronal suture
Anterior fontanel Lamboid suture Biparietal 9.5cm
OA
ROT
ROA
FIGURE 8.2. Various positions in vertex presentation. LOP = left occiput posterior; LOT = left occiput trans-verse; LOA = left occiput anterior; ROP = right occiput posterior; ROT = right occiput transverse; ROA = right occiput anterior.
Before labor 0% effacement
Early effacement 30%
Complete effacement 100%
Complete dilation FIGURE 8.3. Effacement and dilation.
these measurements are used to further define station.
These divisions represent centimeters above and below the ischial spines. Thus, as the presenting fetal part descends from the pelvic inlet toward the ischial spines, the designa-tion is −5, −4, −3, −2, −1, then 0 station. Below the ischial spines, the presenting fetal part passes +1, +2, +3, +4, with +5 station corresponding to the fetal head being visible at the introitus. The clinical significance of the fetal head present-ing at zero station is that the biparietal diameter of the fetal head, the greatest transverse diameter of the fetal skull, is assumed to have negotiated the pelvic inlet.
early dilation, and (2) the active phase of labor, during which more rapid cervical dilation occurs, usually begin-ning at approximately 4 cm.
• The second stage of labor encompasses complete cer-vical dilation through the delivery of the infant.
• The third stage of labor begins immediately after delivery of the infant and ends with the delivery of the placenta.
• The fourth stage of labor is defined as the immediate postpartum period of approximately 2 hours after deliv-ery of the placenta, during which time the patient under-goes significant physiologic adjustment.
Table 8.1 outlines the duration of various stages of labor, as first described in the research by Emmanuel Friedman, and Figure 8.5 represents this information graphically, known as the Friedman curve. New data, derived since the advent of epidural labor analgesia, suggest that the maximum slope of the normal labor curve during active phase may actually be slightly less steep.
MECHANISM OF LABOR
The mechanisms of labor (also known as the cardinal movements of labor [Fig. 8.6]) refer to the changes of the position of the fetus as it passes through the birth canal. The fetus usually descends to where the occipital portion of the fetal head is the lowermost part in the pelvis, and it rotates toward the largest pelvic segment. Because vertex presentation
-3 -5
+5 0
-2 -10 +1 +2 +3
Station (station 0 when fetal vertex at ischial spines)
Not engaged
Biparietal diameter
Pelvic inlet:
sacral promontory and symphysis pubis
Ischial spine
Engaged
FIGURE 8.4. Station and engagement of the fetal head.
The fetal head is said to be engaged at zero station, a crucial functional “landmark” in the labor path.
However, caput succedaneum, cephalohematoma, and molding of the fetal head may mislead the examiner to a greater station than has been obtained.
STAGES OF LABOR
Although labor is a continuous process, it is divided into four functional stages because each has differing physio-logical activities and requires differing management.
• The first stage of labor is the interval between the onset of labor and full cervical dilation (10 cm). The first stage is further divided into two phases: (1) The latent phase of labor encompasses cervical effacement and
occurs in 95% of term labors, the cardinal movements of labor are defined relative to this presentation. To accommodate to the maternal bony pelvis, the fetal head must undergo several movements as it passes through the birth canal. These movements are accomplished by means of the forceful con-tractions of the uterus. These cardinal movements of labor do not occur as a distinct series of movements, but rather as a group of movements that overlap as the fetus accommo-dates and moves progressively through the birth canal.
These movements are 1. Engagement 2. Flexion 3. Descent
4. Internal rotation
