COURSE AND CONDUCT OF LABOR AND DELIVERY ppt

CHAPTER 6

COURSE AND CONDUCT OF LABOR AND DELIVERY 153

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6

COURSE AND CONDUCT

OF LABOR AND DELIVERY

CHAPTER

Labor is the normal process of coordinated, effective involuntary

uterine contractions that lead to progressive cervical effacement

and dilatation and descent and delivery of the newborn and pla￾centa. Near its termination, labor may be augmented by voluntary

bearing-down efforts to assist in delivery of the conceptus.

False labor is characterized by irregular (both in interval and

duration), brief contractions without fundal dominance, cervical

change, or a lower station of the fetal vertex or breech.

Dilatation of the cervix is the diameter of the cervical os ex￾pressed in centimenters (0–10). Effacement is cervical thinning that

occurs before and especially during first stage labor. Effacement of

the cervix is expressed as a percentage of cervical length (normally

2.5 cm) (Figs. 6-1, 6-2). An uneffaced cervix is 0%; one about

0.25 in length is 100% effaced. Effacement and dilatation are caused

by retraction (takeup) of the cervix toward the uterine corpus, not

by pressure of the presenting part.

The initiation of labor in the human is poorly understood.

Labor can be triggered by one or more significant endocrine or

physical changes, for example, abdominal trauma. The onset of la￾bor can occur at any time after well-established pregnancy, but the

likelihood increases as term is approached. Labor can be induced

or stimulated (augmented) by oxytocic agents (e.g., oxytocin or

prostaglandin E2) (Fig. 6-3).

In
10% of gravidas, the fetal membranes rupture before the

onset of labor. This reduces the capacity of the uterus, thickens the

uterine wall, and increases uterine irritability. Labor usually follows.

At term, 90% will be in labor within 24 h after membrane rupture.

If labor does not begin in 24 h, the case must be considered com￾plicated by prolonged rupture of the membranes.

Immediately before or early in labor, a small amount of red￾tinged mucus may be passed (bloody show or mucous plug). This

is a collection of thick cervical mucus often mixed with blood and

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BENSON & PERNOLL’S

154 HANDBOOK OF OBSTETRICS AND GYNECOLOGY

is evidence of cervical dilatation and effacement and, frequently,

descent of the presenting part.

The beginning of true labor is marked by increasingly frequent,

forceful, prolonged, and, finally, regular uterine contractions. Low

backache may precede or accompany the uterine contractions

(pains). Each contraction starts with a gradual buildup of intensity,

and a similar dissipation follows the peak. Normally, the contrac￾tion will be at its height before discomfort is felt. Dilatation of the

lower birth canal almost always will cause deep pelvic or perineal

pain. Nonetheless, occasional nulliparas and some multiparas may

have a brief, virtually pain-free labor.

Labor entails the interaction of the so-called 4Ps.

● The passenger (the fetal size, presentation, position) ● The pelvis (size and shape) ● The powers(effective forces of labor, e.g., uterine contractions) ● The placenta (an obstruction if implanted low in the

uterus)

FIGURE 6-1. Dilatation and effacement of the cervix in a primipara.

FIGURE 6-2. Dilatation and effacement of the cervix in a multipara.

FIGURE 6-3. Production of prostaglandins in human parturition.

(Modified after Liggins.) (From M.L. Pernoll and R.C. Benson, eds. Current Obstetric &

Gynecologic Diagnosis & Treatment, 6th ed. Lange, 1987.)

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BENSON & PERNOLL’S

156 HANDBOOK OF OBSTETRICS AND GYNECOLOGY

Each of these factors, alone or in combination, can make for a

normal or a complicated labor and delivery. For example, if the fe￾tus is large and the pelvis is small, labor may be prolonged or

progress may be impossible despite strong contractions, even with

a placenta normally implanted in the fundus.

NORMAL LABOR

Since, hopefully, the end result of labor is the vaginal delivery of

the fetus, membranes, and placenta, the method of judging its

progress is based on assessments toward that end. The first stage

of labor begins with the onset of labor and ends with complete

FIGURE 6-4. Relationship between cervical dilatation and descent of the

presenting part in a primipara. L, latent phase; A, acceleration phase; M, phase

of maximum slope; D, deceleration phase; and 2, second stage.

(From M.L. Pernoll and R.C. Benson, eds. Current Obstetric & Gynecologic Diagnosis

& Treatment, 6th ed. Lange, 1987.)