Chapter 18 The Second Stage of Labour

The nature of transition and second-stage phases of labour

The second stage of labour is the phase between full dilatation of the cervical os and the birth of the baby. However, most midwives and labouring women are aware of a transitional period between the dilatation, or first stage of labour, and the time when active maternal pushing efforts begin. This period is typically characterised by maternal restlessness, discomfort, desire for pain relief, a sense that the process is never-ending and demands to attendants to get the birth over with as quickly as possible. Some women may experience the urge to push before the cervix is fully dilated, and others may have a lull in activity before the full expulsive nature of the second-stage contractions becomes evident. This latter phenomenon is termed the resting phase of the second stage of labour.

The onset of the second stage of labour is traditionally confirmed with a vaginal examination, however it may not be necessary unless there are signs that the labour is not progressing as anticipated.

Uterine action

Contractions become stronger and longer but may be less frequent. The membranes often rupture spontaneously towards the end of the first stage or during transition to the second stage. Fetal axis pressure increases flexion of the head, which results in smaller presenting diameters, more rapid progress and less trauma to both mother and fetus. If the mother is upright during this time, these processes are optimised.

The contractions become expulsive as the fetus descends further into the vagina. Pressure from the presenting part stimulates nerve receptors in the pelvic floor (this is termed the ‘Ferguson reflex’) and the woman experiences the need to push. This reflex may initially be controlled to a limited extent but becomes increasingly compulsive, overwhelming and involuntary during each contraction. The mother’s response is to employ her secondary powers of expulsion by contracting her abdominal muscles and diaphragm.

Soft tissue displacement

With descent of the head, the soft tissues of the pelvis become displaced.

Anteriorly, the bladder is pushed upwards; this results in the stretching and thinning of the urethra so that its lumen is reduced.

Posteriorly, the rectum becomes flattened into the sacral curve and the pressure of the advancing head expels any residual faecal matter.

The levator ani muscles dilate, thin out and are displaced laterally, and the perineal body is flattened, stretched and thinned.

The fetal head becomes visible at the vulva, advancing with each contraction and receding between contractions until crowning takes place. The head is then born. The shoulders and body follow with the next contraction, accompanied by a gush of amniotic fluid and sometimes of blood. The second stage culminates in the birth of the baby.

Duration of the second stage

The time taken to complete the second stage will vary considerably. Although many maternity units do currently impose routine limits on the duration of the second stage, beyond which medical help should be called, these are not based on good evidence.

Maternal response to transition and the second stage

Pushing

The urge to push may come before the vertex is visible. Traditionally, in order to conserve maternal effort and allow the vaginal tissues to stretch passively, the mother is encouraged to avoid active pushing at this stage. It is now accepted that managed active pushing accompanied by breath holding (the Valsalva manœuvre) has adverse consequences. Whenever active pushing commences, the woman should be encouraged to follow her own inclinations in relation to expulsive effort. Few women need instruction on how to push unless they are using epidural analgesia. Some mothers vocalise loudly as they push.

Position

If the mother lies flat on her back, then vena caval compression is increased, resulting in hypotension. This can lead to reduced placental perfusion and diminished fetal oxygenation and the efficiency of uterine contractions may be reduced.

The mother’s instinctive preference is the primary consideration:

Semirecumbent or supported sitting position, with the thighs abducted. This is the posture most commonly used in Western cultures.

Squatting, kneeling, all fours or standing. Radiological evidence demonstrates an average increase of 1 cm in the transverse diameter and 2 cm in the anteroposterior diameter of the pelvic outlet when the squatting position is adopted.

Left lateral position. An assistant may be required to support the right thigh, which may not be ergonomic. It is an alternative position for women who find it difficult to abduct their hips.

The mechanism of normal labour

As the fetus descends, soft tissue and bony structures exert pressures that lead to descent through the birth canal by a series of movements, called mechanism of labour. During vaginal birth, the fetal presentation, position and size will govern the exact mechanism, as the fetus responds to external pressures. Principles common to all mechanisms are:

Descent takes place.

Whichever part leads and first meets the resistance of the pelvic floor will rotate forwards until it comes under the symphysis pubis.

Whichever part emerges from the pelvis will pivot around the pubic bone.

During the mechanism of normal labour the fetus turns slightly to take advantage of the widest available space in each plane of the pelvis. The widest diameter of the pelvic brim is the transverse; at the pelvic outlet the greatest space lies in the anteroposterior diameter.

At the onset of labour, the most common presentation is the vertex and the most common position either left or right occipitoanterior. When these conditions are met, the way that the fetus is normally situated can be described as follows:

The lie is longitudinal.

The presentation is cephalic.

The position is right or left occipitoanterior.

The attitude is one of good flexion.

The denominator is the occiput.

The presenting part is the posterior part of the anterior parietal bone.

Main movements

Descent

Throughout the first stage of labour the contraction and retraction of the uterine muscles allow less room in the uterus, exerting pressure on the fetus to descend. Following rupture of the forewaters and the exertion of maternal effort, progress speeds up.

Flexion

Pressure exerted down the fetal axis will increase flexion, resulting in smaller presenting diameters that will negotiate the pelvis more easily.

At the onset of labour the suboccipitofrontal diameter (10 cm) is presenting.

With flexion the suboccipitobregmatic diameter (9.5 cm) presents.

The occiput becomes the leading part.

Internal rotation of the head

During a contraction the leading part is pushed downwards onto the pelvic floor. The resistance of this muscular diaphragm brings about rotation.

In a well-flexed vertex presentation the occiput leads and meets the pelvic floor first and rotates anteriorly through one-eighth of a circle.

This causes a slight twist in the neck of the fetus, as the head is no longer in direct alignment with the shoulders.

The anteroposterior diameter of the head now lies in the widest (anteroposterior) diameter of the pelvic outlet.

The occiput slips beneath the subpubic arch and crowning occurs when the head no longer recedes between contractions and the widest transverse diameter (biparietal) is born.

While flexion is maintained, the suboccipitobregmatic diameter (9.5 cm) distends the vaginal orifice.

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Related posts:

1. The Placenta
2. Prolonged Pregnancy and Disorders of Uterine Action
3. Obstetric Emergencies
4. Problems of Pregnancy

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