Overview of Labor

Overview of Labor

Betsy Babb Kennedy

E. Jean Martin

Heather M. Robbins

Physiology of Labor and Birth

Identifying Features of Labor

Labor can be defined medically as regular, progressively intense uterine contractions that, over time, produce cervical effacement and dilation, leading to the development of expulsive forces adequate to move the fetus through the birth canal against the resistance of soft tissue, muscle, and the bony structure of the pelvis.

NOTE: Uterine contractions in the absence of cervical change are not labor.

A number of integrated and sequential biochemical, physiologic, and pharmacologic pathways are thought to exist; it is through these pathways that the term labor is initiated and maintained. The exact mechanism for the onset and maintenance of labor has not yet been fully revealed. Improved experimental laboratory techniques and increasingly sophisticated research approaches in humans are leading to better understanding of the numerous hormonal interactions in human labor and birth; however, these are processes that are not able to be directly investigated.

NOTE: Ancient civilizations believed that the fetus was delivered head first so that it could kick its legs against the top of the uterus and propel itself through the birth canal!1

There is no doubt that, under hormonal influences, the uterus is maintained in a quiescent state throughout most of pregnancy. Certainly, a dramatic physiologic change is involved in taking a pregnancy from the state of relatively low-level antepartum uterine contractility to the coordinated, intense uterine contractility of labor. However, rather than an active process initiated by uterine stimulants, labor may be promoted as a result of the removal of the inhibitory effects of pregnancy on the myometrium.1 In addition, there is substantial evidence from the research that indicates the fetus plays an important role in the timing of labor. In fact, there may be a “parturition cascade” that involves the fetus, the mother, and the placenta.1

Labor Initiation and Maintenance

  • During quiescence, the uterus is maintained by inhibitors such as progesterone, prostacyclin, relaxin, nitric oxide, adrenomedullin, and other substances.

  • In the weeks and days before term, all parts of the uterus undergo preparation for labor and delivery (parturition).2

  • Hormonal mediators from the placenta and maternal and fetal endocrine glands are believed to affect the regulation of the uterine musculature (myometrium).

  • In pregnancy, the lining of the uterus (the endometrium) is referred to as the decidua. Experiencing marked change in thickness and vascularity, the decidua has the capacity to alter hormonal proportions (e.g., estrogen increases over progesterone content) and enzymes and to nourish the embryo. Special decidual cells called macrophages synthesize prostaglandins and another group of compounds called cytokines.3

  • Direct tissue-to-tissue communication occurs among uterine musculature, the decidua, and fetal membranes.3

  • The uterine myometrium consists of thick and thin contractile fibers grouped in bundles. Few intracellular contacts between them exist until late in pregnancy. At that time, areas between muscle fiber cells develop pathways for communication (cell to cell). These pathways are called gap junctions. They are clearly present in great numbers as parturition nears. These efficient cell-to-cell gap junctions serve as channels for the transfer of chemical and electrical signals from one muscle fiber cell to another. Simultaneous contractions of a majority of cells are needed to make an effective contraction. This synchronization of the uterine muscle fibers leads to efficient, coordinated contractions, which soften, thin, and dilate the cervix.2

  • A placental hormone called corticotropin-releasing factor (CRF) is released into maternal circulation early in the second trimester, with concentrations rising significantly as pregnancy advances. CRF production increases the strength of contractions and stimulates production of oxytocin and prostaglandins.4

  • Prostaglandins are chemicals derived from the fetus, amniotic membranes, decidua, and other sources. These prostaglandins, particularly PGF and PGE2, cause smooth muscle contraction and vasoconstriction, soften (“ripen”) cervical tissue, and modulate hormonal activity.3,5

  • Cytokines are an important group of compounds. These compounds have numerous functions in labor physiology, which act either synergistically or antagonistically.

  • Calcium (the calcium ion) is vital for the contractile process in myometrial cells, which depends on the influx of extracellular free calcium. The calcium ion also plays a critical role in transmitting signals of excitation from the myometrial cell membranes to the contractile complex inside the cell.2

NOTE: Agents that block this movement of calcium, called calcium channel blockers (e.g., nifedipine), are in fact used as tocolytic agents for the purpose of suppressing uterine contractility.

  • Once the uterus has been prepared, myometrial activity may be initiated by the fetoplacental unit and the mother, through the secretion of hormones and mechanical stretch of the uterus.1

  • The nonpregnant or very early gravid uterus is not sensitive to oxytocin. However, oxytocin is secreted in pulses of low frequency throughout pregnancy.2

  • As the uterus gradually approaches term, it is thought that the myometrium becomes increasingly responsive to oxytocic hormones, mainly PGE and PGF. Toward the end of pregnancy, the number of oxytocin receptors increases, peaking in the myometrium and decidua in early labor.

  • Secretion of oxytocin seems to be in a pulsatile fashion, even in labor (Figs. 1.1 and 1.2).2

  • Maternal plasma concentrations of endogenous oxytocin are equivalent to a range of 4 to 6 mU/min during the first stage of labor.6

FIGURE 1.1 Distribution of oxytocin receptors in a pregnant human uterus, removed in preterm labor at 34 weeks. Numbers denote oxytocin receptors (OTRs) per unit measurement (fmol/mg DNA). (Reprinted with permission from Fuchs, A. R., & Fuchs, R. [1996]. Physiology and endocrinology of parturition. In Gabbe, S. G., Niebyl, J. R., & Simpson, J. L. [Eds.], Obstetrics: Normal and problem pregnancies [3rd ed., p. 123]. New York: Churchill Livingstone.)

FIGURE 1.2 Diagrammatic representation of the concentration of myometrial oxytocin, the level of oxytocin needed to elicit contractions, and maternal plasma oxytocin levels at the end of gestation and during labor. Oxytocin is secreted in pulses of low frequency. During labor, pulse frequency increases. Fetal secretion of oxytocin can be considerable and may contribute to the oxytocin level reaching the myometrium. PGF production does not increase significantly until labor is in progress and then increases progressively throughout the third stage of labor. OT, oxytocin. (Reprinted with permission from Fuchs, A. R., & Fuchs, R. [1996]. Physiology and endocrinology of parturition. In Gabbe, S. G., Niebyl, J. R., & Simpson, J. L. [Eds.], Obstetrics: Normal and problem pregnancies [3rd ed., p. 124]. New York: Churchill Livingstone.)

Identifying Stages and Phases of Labor

For the sake of description, labor is divided into the following four stages:

Stage I Stage I begins with the onset of regular uterine contractions and lasts until full dilatation of the cervix is achieved. Dilation is the gradual opening of the cervical entrance to the uterus. Stage one may be further divided into two phases:
Early phase—extends from the onset of regular contractions that cause cervical change to the beginning of the active phase, when dilation occurs more rapidly. It usually extends over hours.
Active phase—begins when the laboring woman reaches approximately 6 cm, and ends when the cervix is 10 cm or completely dilated.
Stage II Stage II begins with full cervical dilation (10 cm) and lasts until the baby is born. During this phase, the presenting part of the fetus descends through the maternal pelvis. Stage II may be accompanied by an increase in bloody show, feelings of pressure in the rectum, nausea and vomiting, and desire to push or bear down.
Stage III Stage III is that part of the process after the birth of the baby during which the placenta is delivered.
Stage IV Stage IV is that part of the process after the delivery of the placenta in which the uterus effectively contracts, preventing excessive bleeding. This is a period of adjustment as the mother’s body functions begin to stabilize.

NOTE: A thorough description of the stages of labor may be found in Module 5.

Uterine Contractions

The uterus is composed of three layers of tissue. These layers are arranged as shown in Figure 1.3.

  • Perimetrium—a thick outer membrane covering the uterus.

  • Myometrium—the middle layer that contains special muscle cells called myometrial cells.

  • Endometrium—the innermost layer containing glands and nutrient tissue.

Figure 1.4 illustrates the changes in the uterus and cervix as normal labor progresses.

Under the influence of myometrial contractions, labor progresses with the uterus becoming separated into two distinct parts. The upper portion becomes thicker and more powerful because of shortening and thickening of the myometrial fibers. This prepares the uterus to exert the effort necessary to push the baby out at birth. The lower portion of the uterus becomes thinner, softer, and more relaxed as the myometrial fibers relax and become longer. As a result, the baby can more easily be pushed out at birth.

FIGURE 1.3 The three major tissue layers of the uterus.

FIGURE 1.4 Changes in the uterus and cervix as normal labor progresses. A. Uterus and cervix at term. B. Uterus and cervix early in stage I. C. Uterus and cervix in stage II.

Downward pressure caused by the contraction of the fundal segment is gradually transmitted to the passive lower segment or cervical portion, causing effacement (thinning of the cervix) and dilatation. The cervix is drawn upward and over the baby, allowing the baby to descend into the passageway. The cervix is made up of an inner part called the internal os and an outer part called the external os. Figure 1.5 demonstrates how the internal and external os change position as effacement occurs.

FIGURE 1.5 A. Cervix before effacement begins. B. Effacement in its early phase. C. Effacement with some dilation. D. Complete effacement and dilation.

Contractions have a wave-like pattern that can be divided into segments (Fig. 1.6).

  • Increment—usually makes up the longest part of the contraction.

  • Acme—the shortest, but most intense, part of the contraction.

  • Decrement—a fairly rapid diminishing of the contraction.

Four characteristics of a contraction have been identified:

Jul 10, 2020 | Posted by in NURSING | Comments Off on Overview of Labor
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