Chapter 8 Normal Labor, Delivery, and Postpartum Care
ANATOMIC CONSIDERATIONS, OBSTETRIC ANALGESIA AND ANESTHESIA, AND RESUSCITATION OF THE NEWBORN
Labor is a process that permits a series of extensive physiologic changes in the mother to allow for the delivery of her fetus through the birth canal. It is defined as progressive cervical effacement and dilation resulting from regular uterine contractions that occur at least every 5 minutes and last 30 to 60 seconds.
The role of the obstetrician is to anticipate and manage abnormalities that may occur to either the maternal or the fetal process. When a decision is made to intervene, it must be considered carefully because each intervention carries not only potential benefits but also potential risks. In most cases, the best management may be close observation and, when necessary, cautious intervention.
Anatomic Characteristics of the Fetal Head and Maternal Pelvis
Vaginal delivery necessitates the accommodation of the fetal head to the bony pelvis.
FETAL HEAD
The head is the largest and least compressible part of the fetus. Thus, from an obstetric viewpoint, it is the most important part, whether the presentation is cephalic or breech.
The fetal skull consists of a base and a vault (cranium). The base of the skull has large, ossified, firmly united, and noncompressible bones. This serves to protect the vital structures contained within the brain stem.
The cranium consists of the occipital bone posteriorly, two parietal bones bilaterally, and two frontal and temporal bones anteriorly. The cranial bones at birth are thin, weakly ossified, easily compressible, and interconnected only by membranes. These features allow them to overlap under pressure and to change shape to conform to the maternal pelvis, a process known as “molding.”
Sutures
The membrane-occupied spaces between the cranial bones are known as sutures. The sagittal suture lies between the parietal bones and extends in an anteroposterior direction between the fontanelles, dividing the head into right and left sides (Figure 8-1). The lambdoid suture extends from the posterior fontanelle laterally and serves to separate the occipital from the parietal bones. The coronal suture extends from the anterior fontanelle laterally and serves to separate the parietal and frontal bones. The frontal suture lies between the frontal bones and extends from the anterior fontanelle to the glabella (the prominence between the eyebrows).
Fontanelles
The membrane-filled spaces located at the point where the sutures intersect are known as fontanelles, the most important of which are the anterior and posterior fontanellesClinically, they are even more useful in diagnosing the fetal head position than the sutures.
The posterior fontanelle closes at 6 to 8 weeks of life, whereas the anterior fontanelle does not become ossified until about 18 months. This allows the skull to accommodate the tremendous growth of the infant’s brain after birth.
The anterior fontanelle (bregma) is found at the intersection of the sagittal, frontal, and coronal sutures. It is diamond shaped and measures about 2 × 3 cm, and it is much larger than the posterior fontanelle. The posterior fontanelle is Y- or T-shaped and is found at the junction of the sagittal and lambdoid sutures.
Landmarks
The fetal skull is characterized by a number of landmarks. Moving from front to back, they include the following (Figure 8-2):
Diameters
Several diameters of the fetal skull are important (see Figures 8-1 and 8-2). The anteroposterior diameter presenting to the maternal pelvis depends on the degree of flexion or extension of the head and is important because the various diameters differ in length. The following measurements are considered average for a term fetus:
The transverse diameters of the fetal skull are as follows:
The average circumference of the term fetal head, measured in the occipitofrontal plane, is 34.5 cm.
PELVIC ANATOMY
Bony Pelvis
The bony pelvis is made up of four bones: the sacrum, coccyx, and two innominates (composed of the ilium, ischium, and pubis). These are held together by the sacroiliac joints, the symphysis pubis, and the sacrococcygeal joint. The union of the pelvis and the vertebral column stabilizes the pelvis and allows weight to be transmitted to the lower extremities.
The sacrum consists of five fused vertebrae. The anterior-superior edge of the first sacral vertebra is called the promontory, which protrudes slightly into the cavity of the pelvis. The anterior surface of the sacrum is usually concave. It articulates with the ilium at its upper segment, with the coccyx at its lower segment, and with the sacrospinous and sacrotuberous ligaments laterally.
The coccyx is composed of three to five rudimentary vertebrae. It articulates with the sacrum, forming a joint, and occasionally the bones are fused.
The pelvis is divided into the false pelvis above and the true pelvis below the linea terminalis. The false pelvis is bordered by the lumbar vertebrae posteriorly, an iliac fossa bilaterally, and the abdominal wall anteriorly. Its only obstetric function is to support the pregnant uterus.
The true pelvis is a bony canal and is formed by the sacrum and coccyx posteriorly and by the ischium and pubis laterally and anteriorly. Its internal borders are solid and relatively immobile. The posterior wall is twice the length of the anterior wall. The true pelvis is the area of concern to the obstetrician because its dimensions are sometimes not adequate to permit passage of the fetus.
Pelvic Planes
The pelvis is divided into the following four planes for descriptive purposes:
These planes are imaginary, flat surfaces that extend across the pelvis at different levels. Except for the plane of greatest diameter, each plane is clinically significant.
The plane of the inlet is bordered by the pubic crest anteriorly, the iliopectineal line of the innominate bones laterally, and the promontory of the sacrum posteriorly. The fetal head enters the pelvis through this plane in the transverse position.
The plane of greatest diameter is the largest part of the pelvic cavity. It is bordered by the posterior midpoint of the pubis anteriorly, the upper part of the obturator foramina laterally, and the junction of the 2nd and 3rd sacral vertebrae posteriorly. The fetal head rotates to the anterior position in this plane.
The plane of least diameter is the most important from a clinical standpoint because most instances of arrest of descent occur at this level. It is bordered by the lower edge of the pubis anteriorly, the ischial spines and sacrospinous ligaments laterally, and the lower sacrum posteriorly. Low transverse arrests generally occur in this plane.
The plane of the pelvic outlet is formed by two triangular planes with a common base at the level of the ischial tuberosities. The anterior triangle is bordered by the subpubic angle at the apex, the pubic rami on the sides, and the bituberous diameter at the base. The posterior triangle is bordered by the sacrococcygeal joint at its apex, the sacrotuberous ligaments on the sides, and the bituberous diameter at the base. This plane is the site of a low pelvic arrest.
Pelvic Diameters
The diameters of the pelvic planes represent the amount of space available at each level. The key measurements for assessing the capacity of the maternal pelvis include the following:
The average lengths of the diameters of each pelvic plane are listed in Table 8-1.
TABLE 8-1 AVERAGE LENGTH OF PELVIC PLANE DIAMETERS
| Pelvic Plane | Diameter | Average Length (cm) |
|---|---|---|
| Inlet | True (anatomic) conjugate | 11.5 |
| Obstetric conjugate | 11 | |
| Transverse | 13.5 | |
| Oblique | 12.5 | |
| Posterior sagittal | 4.5 | |
| Greatest diameter | Diagonal conjugate | 12.75 |
| Transverse | 12.5 | |
| Midplane | Anteroposterior | 12 |
| Bispinous | 10.5 | |
| Posterior sagittal | 4.5-5 | |
| Outlet | Anatomic anteroposterior | 9.5 |
| Obstetric anteroposterior | 11.5 | |
| Bituberous | 11 | |
| Posterior sagittal | 7.5 |
Pelvic Inlet
The pelvic inlet has five important diameters (Figure 8-3). The anteroposterior diameter is described by one of two measurements. The true conjugate (anatomic conjugate) is the anatomic diameter and extends from the middle of the sacral promontory to the superior surface of the pubic symphysis. The obstetric conjugate represents the actual space available to the fetus and extends from the middle of the sacral promontory to the closest point on the convex posterior surface of the symphysis pubis.
The transverse diameter is the widest distance between the iliopectineal lines. Each oblique diameter extends from the sacroiliac joint to the opposite iliopectineal eminence.
The posterior sagittal diameter extends from the anteroposterior and transverse intersection to the middle of the sacral promontory.
Plane of Greatest Diameter
The plane of greatest diameter has two noteworthy diameters. The anteroposterior diameter extends from the midpoint of the posterior surface of the pubis to the junction of the 2nd and 3rd sacral vertebrae. The transverse diameter is the widest distance between the lateral borders of the plane.
Plane of Least Diameter (Midplane)
The plane of least diameter has three important diameters. The anteroposterior diameter extends from the lower border of the pubis to the junction of the fourth and fifth sacral vertebrae. The transverse (bispinous) diameter extends between the ischial spines. The posterior sagittal diameter extends from the midpoint of the bispinous diameter to the junction of the fourth and fifth sacral vertebrae.
Pelvic Outlet
The pelvic outlet has four important diameters (Figure 8-4). The anatomic anteroposterior diameter extends from the inferior margin of the pubis to the tip of the coccyx, whereas the obstetric anteroposterior diameter extends from the inferior margin of the pubis to the sacrococcygeal joint. The transverse (bituberous) diameter extends between the inner surfaces of the ischial tuberosities, and the posterior sagittal diameter extends from the middle of the transverse diameter to the sacrococcygeal joint.
PELVIC SHAPES
Based on the general bony architecture, the pelvis may be classified into four basic types (Figure 8-5).
FIGURE 8-5 The four basic pelvic types. The dotted line indicates the transverse diameter of the inlet. Note that the widest diameter of the inlet is posteriorly situated in an android or anthropoid pelvis. The gynecoid pelvis illustrates the location of the sacrosciatic notch, present in all pelvic types.
Gynecoid
The gynecoid pelvis is the classic female type of pelvis and is found in about 50% of women. It has the following characteristics:
These features create a cylindrical shape that is spacious throughout. The fetal head generally rotates into the occipitoanterior position in this type of pelvis.
Android
The android pelvis is the typical male type of pelvis, and it is found in less than 30% of women and has the following characteristics:
This type of pelvis has limited space at the inlet and progressively less space as one moves down the pelvis, owing to the funneling effect of the side walls, sacrum, and pubic rami. Thus, the amount of space is restricted at all levels. The fetal head is forced to be in the occipitoposterior position to conform to the narrow anterior pelvis. Arrest of descent is common at the midpelvis.
Anthropoid
The anthropoid pelvis resembles that of the anthropoid ape. It is found in about 20% of women and has the following characteristics:
The fetal head can engage only in the anteroposterior diameter and usually does so in the occipitoposterior position because there is more space in the posterior pelvis.
ENGAGEMENT
Engagement occurs when the widest diameter of the fetal presenting part has passed through the pelvic inlet. In cephalic presentations, the widest diameter is biparietal; in breech presentations, it is intertrochanteric.
The station of the presenting part in the pelvic canal is defined as its level above or below the plane of the ischial spines. The level of the ischial spines is assigned as “zero” station, and each centimeter above or below this level is given a minus or plus designation, respectively, for a total length of 10 cm.
In most women, the bony presenting part is at the level of the ischial spines when the head has become engaged. The fetal head usually engages with its sagittal suture in the transverse diameter of the pelvis. The head position is considered to be synclitic when the biparietal diameter is parallel to the pelvic plane and the sagittal suture is midway between the anterior and posterior planes of the pelvis. When this relationship is not present, the head is considered to be asynclitic (Figure 8-6).
There is a distinct advantage to having the head engage in asynclitism in certain situations. In a synclitic presentation, the biparietal diameter entering the pelvis measures 9.5 cm; but when the parietal bones enter the pelvis in an asynclitic manner, the presenting diameter measures 8.75 cm. Therefore, asynclitism permits a larger head to enter the pelvis than would be possible in a synclitic presentation.
CLINICAL PELVIMETRY
The diameters that can be clinically evaluated can be assessed at the time of the first prenatal visit to screen for obvious pelvic contractions, although some obstetricians believe that it is better to wait until later in pregnancy when the soft tissues are more distensible and the examination is less uncomfortable and possibly more accurate.
The clinical evaluation is started by assessing the pelvic inlet. The pelvic inlet can be evaluated clinically for its anteroposterior diameter. The obstetric conjugate can be estimated from the diagonal conjugate, which is obtained on clinical examination (see Figure 8-3).
The diagonal conjugate is approximated by measuring from the lower border of the pubis to the sacral promontory using the tip of the second finger and the point where the base of the index finger meets the pubis (Figure 8-7). The obstetric conjugate is then estimated by subtracting 1.5 to 2 cm, depending on the height and inclination of the pubis. Often the middle finger of the examining hand cannot reach the sacral promontory; thus, the obstetric conjugate is considered adequate. If the diagonal conjugate is greater than or equal to 11.5 cm, the anteroposterior diameter of the inlet is considered to be adequate.
The anterior surface of the sacrum is then palpated to assess its curvature. The usual shape is concave. A flat or convex shape may indicate anteroposterior constriction throughout the pelvis.
The midpelvis cannot accurately be measured clinically in either the anteroposterior or transverse diameter. A reasonable estimate of the size of the midpelvis, however, can be obtained as follows. The pelvic side walls can be assessed to determine whether they are convergent rather than having the normal, almost parallel, configuration. The ischial spines are palpated carefully to assess their prominence, and several passes are made between the spines to approximate the bispinous diameter. The length of the sacrospinous ligament is assessed by placing one finger on the ischial spine and one finger on the sacrum in the midline. The average length is 3 fingerbreadths. If the sacrospinous notch that is located lateral to the ligament can accommodate two-and-a-half fingertips, the posterior midpelvis is most likely of adequate dimensions. A short ligament suggests a forward inclination of the sacrum and a narrowed sacrospinous notch (see Figure 8–5, pg 95).
Finally, the pelvic outlet is assessed. This is done by first placing a fist between the ischial tuberosities. An 8.5-cm distance is considered an adequate transverse diameter. The posterior sagittal measurement should also be greater than 8 cm. The infrapubic angle is assessed by placing a thumb next to each inferior pubic ramus and then estimating the angle at which they meet. An angle of less than 90 degrees is associated with a contracted transverse diameter in the midplane and outlet.
Radiologic Assessment of the Pelvis
When an accurate measurement of the pelvis is indicated, nuclear magnetic resonance imaging (MRI) may be used. The advantage of MRI over x-ray or computed tomography (CT) for pelvic assessment is the lack of ionizing radiation exposure.
PREPARATION FOR LABOR
Before actual labor begins, a number of physiologic preparatory events usually occur.
Lightening
Two or more weeks before labor, the fetal head in most primigravid women settles into the brim of the pelvis. In multigravida, this often does not occur until early in labor. Lightening may be noted by the mother as a flattening of the upper abdomen and an increased prominence of the lower abdomen.
False Labor
During the last 4 to 8 weeks of pregnancy, the uterus undergoes irregular contractions that normally are painless. Such contractions appear unpredictably and sporadically and can be rhythmic and of mild intensity. In the last month of pregnancy, these contractions may occur more frequently, sometimes every 10 to 20 minutes, and with greater intensity. These Braxton Hicks contractions are considered false labor in that they are not associated with progressive cervical dilation or effacement. They may serve a physiologic role in preparing the uterus and cervix for true labor.
Cervical Effacement
Before the onset of parturition, the cervix is frequently noted to soften as a result of increased water content and collagen lysis. Simultaneous effacement, or thinning of the cervix, occurs as it is taken up into the lower uterine segment (Figure 8-8B). Consequently, patients often present in early labor with a cervix that is already partially effaced. As a result of cervical effacement, the mucous plug within the cervical canal may be released. The onset of labor may thus be heralded by the passage of a small amount of blood-tinged mucus from the vagina (“bloody show”).
STAGES OF LABOR
There are four stages of labor, each of which is considered separately. These stages in actuality are definitions of progress during labor, delivery, and the puerperium.
The first stage is from the onset of true labor to complete dilation of the cervix. The second stage is from complete dilation of the cervix to the birth of the baby. The third stage is from the birth of the baby to delivery of the placenta. The fourth stage is from delivery of the placenta to stabilization of the patient’s condition, usually at about 6 hours postpartum.
First Stage of Labor
PHASES
The first stage of labor consists of two phases: a latent phase, during which cervical effacement and early dilation occur, and an active phase, during which more rapid cervical dilation occurs (Figure 8-9). Although cervical softening and early effacement may occur before labor, during the first stage of labor, the entire cervical length is retracted into the lower uterine segment.
FIGURE 8-9 Cervical dilation and descent of the fetal head during labor. The first descent curve represents a fetus with a floating presenting part at the onset of labor, whereas the second represents a fetus with the presenting part fixed in the pelvis before labor.
(Modified from Friedman EA: Labor: Evaluation and Management, 2nd ed. East Norwalk, CT, Appleton-Century-Crofts, 1978, p 41.)
LENGTH
The length of the first stage may vary in relation to parity; primiparous patients generally experience a longer first stage than do multiparous patients (Table 8-2). Because the latent phase may overlap considerably with the preparatory phase of labor, its duration is highly variable. It may also be influenced by other factors, such as sedation and stress. The active phase begins when the cervix is 3 to 4 cm dilated in the presence of regularly occurring uterine contractions. The minimal dilation during the active phase of the first stage is nearly the same for primiparous and multiparous women: 1 and 1.2 cm/hour, respectively. If progress is slower than this, evaluation for uterine dysfunction, fetal malposition, or cephalopelvic disproportion should be undertaken.
TABLE 8-2 CHARACTERISTICS OF NORMAL LABOR
| Characteristic | Primipara | Multipara |
|---|---|---|
| Duration of first stage | 6-18 hr | 2-10 hr |
| Rate of cervical dilation during active phase | 1 cm/hr | 1.2 cm/hr |
| Duration of second stage | 30 min to 3 hr | 5-30 min |
| Duration of third stage | 0-30 min | 0-30 min |
MEASUREMENT OF PROGRESS
During the first stage, the progress of labor may be measured in terms of cervical effacement, cervical dilation, and descent of the fetal head. The clinical pattern of the uterine contractions alone is not an adequate indication of progress. After completion of cervical dilation, the second stage commences. Thereafter, only the descent, flexion, and rotation of the presenting part are available to assess the progress of labor.
CLINICAL MANAGEMENT OF THE FIRST STAGE
Certain steps should be taken in the clinical management of the patient during the first stage of labor.
MATERNAL POSITION
The mother may ambulate provided that intermittent monitoring ensures fetal well-being and the presenting part is engaged in patients with ruptured membranes. If she is lying in bed, the lateral recumbent position should be encouraged to ensure perfusion of the uteroplacental unit.
ADMINISTRATION OF FLUIDS
Because of decreased gastric emptying during labor, oral fluids are best avoided. However, fasting results in the more rapid development of ketosis in pregnant women. Placement of a 16- to 18-gauge venous catheter is advisable during the active phase of labor. Recently, it has been shown that giving at least 125 mL/hour of 10% dextrose (D) in normal saline (NS), compared with 5% D/NS or just NS, results in significantly shorter laborsThus, this intravenous route is used to both hydrate the patient with crystalloids and provide calories during labor, to administer oxytocin after the delivery of the placenta, and for the treatment of any unanticipated emergencies.
INVESTIGATIONS
Every woman admitted in labor should have a hematocrit or hemoglobin measurement and a blood clot held in the event that a crossmatch is needed. Blood group, Rhesus (Rh) type, and an antibody screen should be done if these are not known. It is also important to know the hepatitis B status of the mother so that a pediatrician can be notified if the mother is positive. Additionally, a voided urine specimen should be checked for the presence of protein and glucose.
MATERNAL MONITORING
Maternal pulse rate, blood pressure, respiratory rate, and temperature should be recorded every 1 to 2 hours in normal labor and more frequently if indicated. Fluid balance, particularly urine output and intake, should be monitored carefully.
ANALGESIA
Adequate analgesia is important during the first stage of labor (see later in this chapter).
FETAL MONITORING
The fetal heart rate should be evaluated either by auscultation with a De Lee stethoscope, by external monitoring with Doppler equipment, or by internal monitoring with a fetal scalp electrode. In uncomplicated pregnancies, continuous electronic fetal monitoring is not necessary, as several studies have demonstrated that intermittent auscultation of the fetal heart rate, when performed in conjunction with a 1:1 nurse-to-patient ratio, results in comparable outcomes. In patients with no significant obstetric risk factors, the fetal heart rate should be auscultated or the electronic monitor tracing evaluated at least every 30 minutes in the active phase of the first stage of labor and at least every 15 minutes in the second stage of labor. In patients with obstetric risk factors, the fetal heart rate should be auscultated or the electronic monitoring tracing evaluated at least every 15 minutes during the active phase of the first stage of labor (immediately following a uterine contraction), and at least every 5 minutes during the second stage.
UTERINE ACTIVITY
Uterine contractions should be monitored every 30 minutes by palpation for their frequency, duration, and intensity. For high-risk pregnancies, uterine contractions should be monitored continuously along with the fetal heart rateThis can be achieved electronically using either an external tocodynamometer or an internal pressure catheter in the amniotic cavity. The latter is particularly of value when a patient’s labor is being augmented with oxytocin (Pitocin).
VAGINAL EXAMINATION
During the latent phase, particularly when the membranes are ruptured, vaginal examinations should be done sparingly to decrease the risk for an intrauterine infection. In the active phase, the cervix should be assessed about every 2 hours to determine the progress of labor.
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