Maternal and Fetal Response to Labor

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

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.

Passage involves

  • the size of the pelvis
  • the shape of the pelvis
  • the ability of the cervix to dilate and the vagina to stretch
Passenger involves

  • fetal size, particularly the fetal head
  • fetal attitude, which describes the relation of the fetal head, shoulder, and legs to one another
  • fetal lie, which refers to the relationship of the long axis (|) of the fetus to the long axis (|) of the mother
  • fetal presentation, which describes that part of the fetus entering the pelvis first
  • fetal position, which refers to the direction toward which the presenting part is pointing—front, side, or back of the maternal pelvis
Powers involve

  • the frequency, duration, and intensity of uterine contractions
  • abdominal pressures resulting from pushing, which occur in stage II of labor
Psyche involves

  • the mother’s physical, emotional, and intellectual preparation
  • her previous childbirth experiences
  • her cultural attitude
  • support from significant people in the mother’s life

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

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.

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:

FIGURE 2.3 Pelvic planes.

Clinical pelvimetry

  • Estimation of pelvic shapes and dimensions by the examiner
  • Wide margin of error depending on the examiner’s skill
X-ray pelvimetry

  • Potential fetal exposure to low-dose radiation
  • Can provide critical pelvic diameters not otherwise obtainable
  • Sometimes used in breech presentations
  • Has been replaced by other pelvic imaging methods

  • Uses sound waves, not ionizing energy
  • Not useful for evaluating maternal pelvic measurement
  • Useful for precisely measuring fetal biparietal diameters and fetal head circumference
Computed tomographic (CT) pelvimetry (CT scanning)

  • Has replaced x-ray pelvimetry at many institutions
  • Accuracy is improved over conventional x-ray pelvimetry
  • Involves a lower fetal radiation exposure than x-ray
  • Maternal movement during procedure needs to be minimal to prevent distortion
  • Expense is comparable to that of conventional x-ray
Magnetic resonance imaging

  • Offers accurate pelvic measurements and complete fetal imaging
  • Has the potential to aid in diagnosing soft tissue dystocia and obstructed labor
  • Use is limited by expense, length of time needed to obtain the study, and availability of equipment

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.

Column A Column B
1. _______________Lie
2. _______________Attitude
3. _______________Position
4. _______________Presentation
a. Relationship of the long axis of the fetus to the long axis of the mother
b. That part of the fetal body that is entering the pelvis
c. The relationship of fetal parts to one another
d. The direction toward which the presenting part is pointing with respect to the front, side, or back of the mother’s pelvis

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:




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

Relationships Between the Fetus and Pelvis

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).

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Jul 10, 2020 | Posted by in NURSING | Comments Off on Maternal and Fetal Response to Labor
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