Maternal and Fetal Response to Labor
E. Jean Martin
Betsy Babb Kennedy
Heather M. Robbins
Part 1 Identifying Features of the Pelvis That Make It Adequate for Labor
Objectives
As you complete Part 1 of this module, you will learn:
Critical factors involved in labor
The significance of each type of pelvis to the birth process
Features of the pelvis that affect labor
Limitations of pelvic evaluation methods
Key Terms
When you have completed Part 1 of this module, you should be able to recall the meaning of the following terms. You should also be able to use the terms when consulting with other health professionals. The terms are defined in this module or in the glossary at the end of this book.
fetal attitude
fetal lie
fetal position
fetal presentation
molding
pelvic planes
Features of the Pelvis That Make It Adequate for Labor
There are four factors, often referred to as the “four Ps,” that affect the progress of labor.
|
For labor to progress smoothly there must be adaptations of both fetal and maternal factors. Abnormalities of any of these critical factors can mean risk for baby, mother, or both.
What makes a pelvis adequate for labor?
The size and the shape of the pelvis make it adequate for labor. The female pelvis is uniquely suited to the demands of childbearing. However, not all women possess the same type of pelvis. The following four classic types of pelves are based on differences in shapes, diameters, and angles (Display 2.1).1,2,3 In clinical practice, consistent prediction of a successful vaginal delivery cannot be done based on pelvis shape classification.4
Display 2.1 Classic Types of Pelves
|
A woman can have a pelvis that has a combination of characteristics from these classic types. Mixed architectural features are encountered frequently in clinical practice.5
In obstetrics, the pelvis is divided into the following parts (Fig. 2.1):
False pelvis—where there is ample room
2.True pelvis—which contains important narrow dimensions through which the fetus must pass
There is a ridge that provides an imaginary dividing line between the two areas. This ridge is the boundary for the inlet to the true pelvis.
 FIGURE 2.1 False pelvis and true pelvis. |
NOTE: The false pelvis has no obstetric significance, whereas the true pelvis has great significance.
The true pelvis can be divided into three key areas (Fig. 2.2):
Inlet
Pelvic cavity, which extends from the inlet to the outlet
Outlet
 FIGURE 2.2 A. Front view of pelvis. B. Side view of pelvis. |
The pelvic planes are imaginary flat surfaces passing across parts of the true pelvis at different levels. Three important planes are shown in Figure 2.3.
How is the adequacy of the pelvis evaluated?
The relationship of the fetal size to the pelvis must be evaluated. This relationship changes depending on the forces and stages of labor, and positioning of the mother can bring about subtle changes in one or two pelvic dimensions (e.g., the McRoberts maneuver). Dynamic changes
in the fetal head, thorax, and abdomen occur as the pelvic passageway is negotiated during descent. Efforts to predict cephalopelvic disproportion have included the following1:
in the fetal head, thorax, and abdomen occur as the pelvic passageway is negotiated during descent. Efforts to predict cephalopelvic disproportion have included the following1:
 FIGURE 2.3 Pelvic planes. |
|
Imaging studies, although readily defining values for the parameters of the true bony pelvis, have not been shown to consistently predict women at risk for cephalopelvic disproportion.5 Radiographic studies are generally avoided during pregnancy because of the theoretical risk of radiation exposure to the fetus. Clinical pelvimetry is a skill, yet still may not allow prediction of the course or outcomes of delivery. A trial of labor is commonly used to determine if the woman’s pelvis is adequate for delivery of the baby.
Additional considerations include the following:
Except for some relaxation of the pelvic joints because of hormonal influences, the bones of the pelvis cannot expand.
The soft tissues of the birth canal (the cervix and pelvic floor musculature) provide resistance during labor. The cervix undergoes biochemical changes that increase its elasticity. The musculature of the pelvic floor facilitates rotation and flexion for the fetal head.
The relationship of the fetal head size to the pelvis is important.
The fetal head has the ability to change shape to fit through the pelvis. This ability of the head to change shape is called molding.
Because of the tilt of the pelvis, the fetus descends through this pathway during labor and birth, as shown in Figure 2.4.
 FIGURE 2.4 Pathway of fetal descent. |
Practice/Review Questions
After reviewing Part 1, answer the following questions.
1. List four critical factors involved in the labor process.
a. __________________________
b. __________________________
c. __________________________
d. __________________________
2. Match the definition in Column B with the correct term in Column A.
|
3. List the four main types of pelves.
a. __________________________
b. __________________________
c. __________________________
d. __________________________
4. The pelvis best suited for labor and birth is the __________________________ pelvis. This type of pelvis is found in __________________________% of women.
5. The pelvis that has narrow dimensions and is likely to result in labor stopping or a forceps delivery is the __________________________pelvis. This type of pelvis is found in __________________________% of women.
6. Match the areas of the pelvis with the correct numbers in the diagram.
__________________________a. Inlet
__________________________b. False pelvis
__________________________c. True pelvis
__________________________d. Outlet
__________________________e. Plane of least dimensions
|
7. The true pelvis is made up of three key planes called:
a.
b.
c.
8. The planes of the true pelvis are critical because: ______
9. Name two ways in which the female pelvis is evaluated for adequacy.
a. __________________________
b. __________________________
10. Explain how it is possible for the fetal head to fit through the rigid, bony pelvis.
Practice/Review Answer Key
Passage
Passenger
Power
Psyche
a
c
d
b
Gynecoid
Android
Platypelloid
Anthropoid
Gynecoid; 50
Android; 20
1
2
5
3
4
Least dimensions
Greatest dimensions
Pelvic inlet
The fetus must pass through these areas, some of which are narrow.
X-ray
Pelvic examination
The head flexes and the bones of the scalp mold somewhat.
Part 2 Identifying Relationships Between the Fetus and Pelvis
Objectives
As you complete Part 2 of this module, you will learn:
Relationships of the fetal position and presenting part to the outcome of labor
Landmarks used to identify the position of the fetus
Features of the pelvis that affect labor
How to determine and describe fetal lie, presentation, attitude, and position
Key Terms
When you have completed Part 2 of this module, you should be able to recall the meaning of the following terms. You should also be able to use the terms when consulting with other health professionals. The terms are defined in this module or in the glossary at the end of this book.
biparietal diameter
denominator
fetal anencephaly
fetal hydrocephaly
fontanelle
grand multiparity
hydramnios
mentum
occiput
placenta previa
prematurity
sinciput
suture
vertex
Relationships Between the Fetus and Pelvis
How does the fetal passenger accommodate to the pelvis during labor?
The position of the fetus as the mother is ready to go into labor largely determines how smoothly the labor and delivery will progress. The fetal head is the largest part of the baby and is composed of both fixed and flexible parts. Becoming familiar with the parts of the fetal skull is essential because the identification of certain landmarks will assist you when performing vaginal examinations to determine the mother’s labor progress. The skull consists of three major divisions (Fig. 2.5):
Face
Back of the skull
Cranium, or top of the skull
 FIGURE 2.5 Major divisions of the fetal skull. |
 FIGURE 2.6 Molding of the fetal head in different cephalic presentations. |
The bones of the face and the back of the skull are fused and fixed, but the cranium consists of several large bones that are not fused together at the time of birth. This permits the shape of the head to change somewhat as the fetus passes through the narrow, rigid pelvis (Fig. 2.6).
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
