Normal Labor, Delivery, and Postpartum Care: Anatomic Considerations, Obstetric Analgesia and Anesthesia, and Resuscitation of the Newborn

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 lambdoidal 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 fontanelles. Clinically, they are even more useful than the sutures for determining the fetal head position.

The posterior fontanelle closes at 6 to 8 weeks of life, whereas the anterior fontanelle does not become ossified until approximately 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, measures approximately 2 × 3 cm, and 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. From front to back, they include the following ( Figure 8-2 ):

• 1

  Nasion: the root of the nose

• 2

  Glabella: the elevated area between the orbital ridges

• 3

  Sinciput (brow): the area between the anterior fontanelle and the glabella

• 4

  Anterior fontanelle (bregma): diamond-shaped

• 5

  Vertex: the area between the fontanelles and bounded laterally by the parietal eminences

• 6

  Posterior fontanelle (lambda): Y- or T-shaped

• 7

  Occiput: the area behind and inferior to the posterior fontanelle and lambdoid sutures

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. It is important because the various diameters differ in length. The following measurements are considered average for a term fetus:

• 1

  Suboccipitobregmatic (9.5 cm): the presenting anteroposterior diameter when the head is well flexed, as in an occipitotransverse or occipitoanterior position; it extends from the undersurface of the occipital bone at the junction with the neck to the center of the anterior fontanelle.

• 2

  Occipitofrontal (11 cm): the presenting anteroposterior diameter when the head is deflexed, as in an occipitoposterior presentation; it extends from the external occipital protuberance to the glabella.

• 3

  Supraoccipitomental (13.5 cm): the presenting anteroposterior diameter in a brow presentation and the longest anteroposterior diameter of the head; it extends from the vertex to the chin.

• 4

  Submentobregmatic (9.5 cm): the presenting an­­teroposterior diameter in face presentations; it extends from the junction of the neck and lower jaw to the center of the anterior fontanelle.

The transverse diameters of the fetal skull are as follows:

• 1

  Biparietal (9.5 cm): the largest transverse diameter; it extends between the parietal bones.

• 2

  Bitemporal (8 cm): the shortest transverse diameter; it extends between the temporal bones.

The average circumference of the term fetal head, measured in the occipitofrontal plane, is 34.5 cm.

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 edge of the pelvic inlet). 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 ( Table 8-1 and Figures 8-3 and 8-4 ):

• 1

  The pelvic inlet

• 2

  The plane of greatest diameter

• 3

  The plane of least diameter

• 4

  The pelvic outlet

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 second and third 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:

• 1

  The obstetric conjugate of the inlet

• 2

  The bispinous diameter of the midplane

• 3

  The bituberous diameter of the outlet

• 4

  The anteroposterior sagittal diameter of the outlet

The average lengths of the diameters of each pelvic plane are listed in Table 8-1 .

Pelvic Inlet

The pelvic inlet has five important diameters (see 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 (see Figure 8-4 ) extends from the midpoint of the posterior surface of the pubis to the junction of the second and third 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 (see 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 (not listed) extends from the middle of the transverse diameter to the sacrococcygeal joint.

Gynecoid

The gynecoid pelvis is the classic female type of pelvis and is found in approximately 50% of women. It has the following characteristics:

• 1

  Round at the inlet, with the widest transverse diameter only slightly greater than the anteroposterior diameter

• 2

  Sidewalls straight

• 3

  Ischial spines of average prominence

• 4

  Large sacrospinous notch (depicted in Figure 8-5 )

• 5

  Well-curved sacrum

• 6

  Spacious subpubic arch with an angle of approximately 90 degrees

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. It is found in less than 30% of women and has the following characteristics:

• 1

  Triangular inlet with a flat posterior segment and the widest transverse diameter closer to the sacrum than in the gynecoid type

• 2

  Convergent sidewalls with prominent spines

• 3

  Shallow sacral curve

• 4

  Long and narrow (small) sacrospinous notch (noted in Figure 8-5 in a gynecoid pelvis)

• 5

  Narrow subpubic arch

This type of pelvis has limited space at the inlet and progressively less space as the fetus moves down the pelvis, because of the funneling effect of the sidewalls, 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 approximately 20% of women and has the following characteristics:

• 1

  A much larger anteroposterior than transverse diameter, creating a long, narrow oval at the inlet

• 2

  Sidewalls that do not converge

• 3

  Ischial spines that are not prominent but are close, because of the overall shape

• 4

  Variable, but usually posterior, inclination of the sacrum

• 5

  Small sacrospinous notch

• 6

  Narrow, outwardly shaped subpubic arch

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.

Platypelloid

The platypelloid pelvis is best described as being a flattened gynecoid pelvis. It is found in only 3% of women, and it has the following characteristics:

• 1

  A short anteroposterior and wide transverse diameter, creating an oval-shaped inlet

• 2

  Straight or divergent sidewalls

• 3

  Posterior inclination of a flat sacrum

• 4

  A wide bispinous diameter

• 5

  Long but small sacrospinous notch

• 6

  A wide subpubic arch

The overall shape is that of a gentle curve throughout. The fetal head has to engage in the transverse diameter.

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 for pelvic assessment is the lack of ionizing radiation exposure.

Indications

• 1

  Clinical evidence or obstetric history suggestive of pelvic abnormalities

• 2

  A history of pelvic trauma

It should always be questioned whether the results obtained by radiologic assessment will have sufficient influence on the patient's management to make the investigation worthwhile.

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 the internal os of the cervix is taken up into the lower uterine segment ( Figure 8-8, B ). 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”).

Oxytocin Infusion

Several principles should be followed when oxytocin is used to induce or augment labor:

• 1

  Oxytocin must be given intravenously to allow it to be discontinued quickly if a complication such as uterine hypertonus or fetal distress develops. Because oxytocin has a half-life of 3 to 5 minutes, its physiologic effect will diminish quickly (within 15 to 30 minutes) after discontinuation.

• 2

  A dilute infusion must be used and “piggybacked” into the main intravenous (IV) line so that it can be stopped quickly if necessary, without interrupting the main IV route.

• 3

  The drug is best infused with a calibrated infusion pump that can be easily adjusted to deliver the required infusion rate accurately.

• 4

  The induction of labor for a specific indication generally should not exceed 72 hours. In patients with a low Bishop score, it is not unusual for an induction to progress slowly. If the cervix effaces and dilates, it is recommended that the membranes be ruptured on the third day. If adequate progress is not made within 12 hours of rupturing the membranes, a cesarean delivery may be performed.

• 5

  If adequate labor is established, the infusion rate and the concentration may be reduced, especially during the second stage of labor. Adherence to this principle avoids the risks of hyperstimulation and fetal distress, which frequently occur once labor has been established.

Substantial variation exists regarding the initial dose, incremental dose, and time interval between dose increments when oxytocin is used for labor induction and augmentation. Well-performed clinical studies have supported both low-dose (1 to 30 mU/min) and high-dose (4 to 40 mU/min) protocols, as shown in Table 8-4 . It is not surprising that many protocols use moderate doses of oxytocin. Generally, intervals between dose increments should be no less than 20 minutes to permit time for steady-state plasma levels of oxytocin to be achieved and to prevent an increased risk of uterine hyperstimulation.

First Stage of Labor as Defined by the Friedman Labor Curve

There is currently controversy about how to define the progress of normal labor. Because the purpose of this chapter is to define normal labor, we will use the standard definitions provided by Friedman in 1978 ( Figure 8-9 ). The Friedman labor curve, as well as a recently suggested modification, is covered in Chapter 11 .