Uterine muscle in labour has the unique property of contraction and retraction (Fig. 37.1). Following each contraction, the muscle fibres do not completely relax but retain some of the shortening of contraction. This is called retraction and leads to the progressive shortening and thickening of the upper uterine segment (UUS) and the diminishing of the uterine cavity to accommodate the descending fetus. A physiological ridge forms between the UUS and the lower uterine segment (LUS), known as a retraction ring (see Ch. 44 for the abnormal Bandl’s ring which is an exaggerated pathological retraction ring that develops in obstructed labour and becomes visible above the symphysis pubis).

Effacement or ‘taking up’ of the cervix is the term used to describe the gradual mergence of the cervix into the LUS. In primigravidae, this process is sometimes complete before the onset of labour and before dilatation of the external cervical os occurs (Fig. 37.2A). In multigravidae, a perceptible cervical canal remains until labour is well established, a finding midwives refer to as a ‘multip’s os’. During labour there is dilatation of the external os until it is large enough for the widest diameter of the presenting part to pass through (Fig. 37.2B). In a fetus at term presenting by the vertex, the diameter of the cervix would normally have to reach 10 cm but this would be less in a fetus with a smaller head. As the cervix begins to dilate, the operculum or mucous plug formed in pregnancy is lost around the time of the commencement of labour and the woman will notice a mucoid discharge, which may be blood-stained. This is termed the show. The blood originates from ruptured capillaries when the lining of the cervix is stretched or where the chorion has become detached from the dilating cervix.

Besides uterine action, there are mechanical forces that aid dilatation of the cervix. As the lower uterine segment is stretched, the chorion is detached from it. In normal labour the increased intrauterine pressure during contractions forces a well-flexed head snugly against the cervix, trapping a small amount of amniotic fluid in front of the head separate from the rest of the fluid surrounding the body of the fetus. This small sac of fluid is known as the forewaters and the rest of the fluid as the hindwaters. The forewaters bulge through the cervix, becoming more tense during contractions (Fig. 37.2A-4/B-2). This separation of the forewaters from the larger volume of the hindwaters keeps the membranes intact during the first stage of labour, providing a barrier against ascending infection.

When the membranes remain intact, the pressure of each contraction is exerted on the fluid and, as fluid is not compressible, pressure is equalised throughout the uterus. This is known as general fluid pressure (Fig. 37.3). If the membranes are ruptured and amniotic fluid is reduced, contraction pressure is applied directly to the fetus. The placenta is compressed between the uterine wall and the fetus, which further reduces the oxygen supply to the fetus. Therefore, there are two good reasons for maintaining intact membranes: to reduce the risk of infection and to maintain a good oxygen supply to the fetus. In a systematic review of 14 studies (4893 women) exploring amniotomy in labour, Smyth et al (2007) did not find any evidence of a statistical difference in length of first stage of labour, maternal satisfaction with childbirth experience or low Apgar score of < 7 at 5 min. However, amniotomy was associated with an increased risk of delivery by caesarean section compared to women in the control group. The physiological moment for the membranes to rupture is when the cervix is fully dilated and no longer able to support the forewaters and the force of the uterine contractions reaches maximum. The evidence remains clear that routine amniotomy during the first stage of labour can never be justified as standard care in normal low-risk women (NICE 2007, Smyth et al 2007).

During each contraction, the force of the fundal contraction is transmitted to the upper pole of the fetus, down the long axis of the fetal spine, causing increasing flexion of the head. This ensures that the smallest possible circumference, which is the circular vertex, is applied to the circular cervical os. This is known as fetal axis pressure (Fig. 37.4).

The latent phase, which lasts until cervical dilatation is about 3–4 cm, can take 6–8 h in a primigravida. The active phase occurs with rapid dilatation of the cervix at about 1 cm/h in a primigravida and 1.5 cm/h in a multigravida. The length of established first stage of labour varies between women. First labours last on average 8 h and are unlikely to last over 18 h. Second and subsequent labours last on average 5 h and are unlikely to last over 12 h (NICE 2007). Plotting the rate of cervical dilatation has been commonly carried out in labour (a cervicograph), an average duration is indicated in Figure 37.5.

Childbearing experience for women encompasses both a biological and a social event. The midwife should carefully consider these aspects when planning care with the labouring woman. This should include the following:

The physiological aspects of care in labour include assessing progress, positioning of the woman, nutrition and hydration and monitoring the condition of woman and fetus.

Most of the aims of abdominal examination, considered by Viccars (2003) in the context of antenatal care, apply equally well to labour. These are to assess fetal size and well-being, to diagnose the location of fetal parts—in particular, lie, presentation, position and engagement—and to detect any deviation from normal. Abdominal examination should always be carried out prior to performing a vaginal examination (if required) and repeated abdominal examinations can be used to assess descent of the presenting part.

Vaginal examination has been routine practice for generations to assess progress in labour. The rationale for why vaginal examination is required in a low-risk situation is still unclear, with evidence to support it being unnecessary and a traumatic experience for women (NICE 2007). With good physiological knowledge of labour, healthcare professionals can use their skills in deciphering progress rather than using intervention. The purple line which creeps up the ‘natal cleft’ (cleavage between the buttocks) can be used as a measure of cervical dilatation (Byrne & Edmonds 1990). Hobbs (2003) proposes that this non-intervention method would be a useful tool to adopt in low-risk situations. At the start of labour the ‘purple-red line’ begins at the anal margin and gradually creeps up the anal cleft (much like mercury in a thermometer) as cervical dilatation occurs. Vaginal examinations in low-risk women are only necessary if they add further information to the decision-making process of the women’s care as these examinations can ultimately cause more distress and discomfort (NICE 2007). Health professionals should use their clinical judgement along with a fully informed decision from the woman prior to conducting a vaginal examination. Low-risk women with a strong history of pre-labour spontaneous rupture of membranes should not be offered a speculum examination routinely, but if there is any uncertainty then a speculum examination should be offered. However, if no uterine contractions are present then a digital vaginal examination should be avoided (NICE 2007).

It is acknowledged that vaginal examinations are not always required in labour. The following rationale for conducting a vaginal examination in labour are suggested: