21.1 Introduction

In this chapter, we shall deal with the mechanism of normal labour, how to diagnose labour and the influence the power (uterine activity), passage (maternal pelvis) and passenger (fetus) have on this process. What are the signs of true labour? What do the guidelines say regarding false labour pains, causes of onset of labour, normal physiology, stages of labour, mechanism of labour clinical course of stages of labour and its management?

We shall build on this knowledge and discuss abnormal patterns of labour, the causes and management options for the treatment of dysfunctional labour.

21.2 Normal Labour

Normal labour is characterised by the onset of regular contractions associated with cervical effacement and dilatation with progressive descent of the presenting part.

A baby presenting by the vertex will generally enter the pelvis with the occiput in the lateral position owing to the fact that the transverse diameter is the largest pelvic inlet diameter. As the presenting part descends through the pelvis, it rotates so that the occiput moves into the anterior position. The fetal spine is connected to the back of the skull, and therefore pressure directed along the spine causes flexion of the fetal head as descent and rotation occur. On passing through the outlet, delivery of the head is achieved by extension. Restitution occurs when the head realigns with the shoulders. External rotation occurs as the shoulders rotate from the transverse position at the inlet to the A/P position at the outlet.

Normal labour begins spontaneously at 37–42 weeks, progresses at an acceptable rate and results in the spontaneous vaginal delivery of a live undistressed neonate in the occipitoanterior position [1–3] (Fig. 21.1).

21.3 Dysfunctional Labour

Dysfunctional labour refers to abnormal labour patterns. The first stage of labour is considered abnormal when cervical dilatation rate is less than 1 cm per hour during active labour. The second stage is considered dysfunctional if there is failure to deliver the fetus within 1 h of commencement of active pushing [1–3].

Dysfunctional labour may be due to problems with the fetus (passenger), problems with maternal or soft tissues (passage) or disorders of the uterine activity (power). Each of these will be addressed later in this chapter.

Labour wards may not be the appropriate environment for women in the latent phase.Women value home assessment in the latent phase, and this can reduce the number of visits to hospitals [4].

The duration of the latent phase is particularly difficult to measure, as women experience the onset of labour in a variety of different ways [5].

A long latent phase can often be a discouraging and exhausting experience, and women need good consistent psychological support.

21.4 Common Definitions [1–3]

Lie—this refers to the relationship between the longitudinal axis of the uterus and the longitudinal axis of the fetus. This is generally longitudinal but may be transverse or oblique.

Presenting part—this is the portion of the fetus felt on vaginal examination.

Position—this is the relationship between a defined area of the presenting part (known as the dominator) and the mother’s pelvis.

Station—this refers to the level of the presenting part in relation to the ischial spines.

Attitude—this refers to the relationship of the fetal head and limbs to the fetal trunk. The attitude is generally one of flexion.

Vertex—this is the area bounded by the anterior fontanelle (bregma), posterior fontanelle and the biparietal eminences.

Occiput—this is the area below the posterior fontanelle.

Sinciput—this is the area in front of the anterior fontanelle. This is divided into the brow (area between the anterior fontanelle and the root of the nose) and the face (area below the root of the nose).

21.5 Anatomical and Physiological Considerations of the Pelvis and Fetus

21.5.1 The Passenger

The cranium is made up of five bones: two parietal bones, two frontal bones and the occiput. Suture lines separate the skull bones. The sagittal suture separates the two parietal bones, and the frontal suture separates the two frontal bones, while the lambdoid suture separates the occipital bone and the parietal bones (Figs. 21.2 and 21.3).

The anterior fontanelle is formed by the junction of the sagittal, frontal and two coronal sutures and is diamond-shaped. The posterior fontanelle is formed by the junction of the sagittal and two lambdoid sutures and is Y-shaped.

The fetal skull has been divided into areas to facilitate the description of the presenting part.

21.6 What Factors Influence the Diameter of the Presenting Part?

The portion of the fetal skull that presents in labour is dependent on the degree of flexion of the fetal head.

During the course of normal labour, the vertex presents, and due to the insertion of the spine posteriorly, flexion of the fetal head results. The widest transverse diameter in this position is the biparietal diameter (9.5 cm). The sagittal diameter tends to be the suboccipitobregmatic diameter (9.5 cm, below the occiput to the centre of the anterior fontanelle).

If the head fails to flex, the resulting diameter is the occipitofrontal (11–12 cm, occiput to the root of the nose).

Further head extension results in the brow presentation and the mentovertical diameter (14 cm, chin to the centre of the sagittal suture).

Continued extension results in a face presentation and submentobregmatic diameter (9.5 cm, angle between the neck, chin and the centre of the anterior fontanelles).

Malformations may also influence the size of the presenting part. Examples of this will include anomalies such as hydrocephalus or large space occupying lesions such as teratomas.

21.6.1 The Passages

The female pelvis consists of a pair of innominate bones (pubis, ischium and ilium) joined anteriorly by the pubic symphysis while articulating with the sacrum and coccyx posteriorly.

The brim of the pelvis is comprised of the upper border of the pubic symphysis, iliopectineal line, ala of the sacrum and sacral promontory. The brim of the normal gynaecoid pelvis is almost round although the sacrum intrudes posteriorly. The transverse diameter of the brim generally measures 13.5 cm and the AP diameter 11 cm.

The midpelvis is almost circular in outline and is bordered by the apex of the pubic symphysis, the ischial spines, the sacrospinous ligament and the tip of the sacrum. Contained within the midcavity is the plane of least dimension which measures 10.5 cm in its transverse plane at the level of the ischial spines. It is at this level that arrest of labour commonly occurs.

The outlet is diamond shaped and is bounded by the subpubic arch, ischial tuberosities, sacrotuberous ligaments and the coccyx.

It is said that the plane and the brim makes an angle of 55° with the horizontal in the erect position [1–3] (Figs. 21.4 and 21.5).