Chapter 21 Malpositions of the Occiput and Malpresentations

Occipitoposterior positions

Occipitoposterior positions are the most common type of malposition of the occiput and occur in approximately 10% of labours. A persistent occipitoposterior position results from a failure of internal rotation prior to delivery. The vertex is presenting, but the occiput lies in the posterior rather than the anterior part of the pelvis. As a consequence, the fetal head is deflexed and larger diameters of the fetal skull present.

Causes

The direct cause is often unknown, but it may be associated with an android- or anthropoid-shaped pelvis.

Antenatal diagnosis

Abdominal examination

Inspection

There is a saucer-shaped depression at or just below the umbilicus.

Palpation

The back is difficult to palpate; it is well out to the side.

Limbs can be felt on both sides of the midline.

The head is usually high.

The occiput and sinciput are on the same level.

Auscultation

The fetal heart can be heard in the midline or in the flank.

Diagnosis during labour

The woman may complain of continuous and severe backache worsening with contractions.

Spontaneous rupture of the membranes may occur at an early stage of labour.

Contractions may be incoordinate.

There is slow descent of the head, even with good contractions.

The woman may have a strong desire to push early in labour.

Vaginal examination

The findings will depend upon the degree of flexion of the head; locating the anterior fontanelle in the anterior part of the pelvis is diagnostic. The direction of the sagittal suture and location of the posterior fontanelle will help to confirm the diagnosis.

Management of labour

Labour with a fetus in an occipitoposterior position can be long and painful. The deflexed head does not fit well onto the cervix and therefore does not produce optimal stimulation.

Management is described in Box 21.1.

Box 21.1 Management of labour: occipitoposterior positions

First stage of labour

• Assist with pain management techniques, such as massage, changes of posture and position. Offer pharmacological pain control methods as appropriate

• Prevent dehydration and ketosis

• Correct any incoordinate uterine action or ineffective contractions with an oxytocin infusion

• The urge to push long before the cervix has become fully dilated may be eased by a change in position and the use of breathing techniques or nitrous oxide and oxygen to enhance relaxation

Second stage of labour

• Full dilatation of the cervix may need to be confirmed by a vaginal examination

• Encourage the woman to remain upright

• If contractions are weak and ineffective, an oxytocin infusion may be commenced

• The length of the second stage of labour is increased when the occiput is posterior, and there is an increased likelihood of operative delivery

Mechanism of the right occipitoposterior position (Long rotation)

See Figures 21.1–21.4 and Table 21.1.

Flexion. Descent takes place with increasing flexion. The occiput becomes the leading part.

Internal rotation of the head. The occiput reaches the pelvic floor first and rotates forwards three-eighths of a circle along the right side of the pelvis to lie under the symphysis pubis. The shoulders follow, turning two-eighths of a circle from the left to the right oblique diameter.

Crowning. The occiput escapes under the symphysis pubis and the head is crowned.

Extension. The sinciput, face and chin sweep the perineum and the head is born by a movement of extension.

Restitution. The occiput turns one-eighth of a circle to the right and the head realigns itself with the shoulders.

Internal rotation of the shoulders. The shoulders enter the pelvis in the right oblique diameter; the anterior shoulder reaches the pelvic floor first and rotates forwards one-eighth of a circle to lie under the symphysis pubis.

External rotation of the head. At the same time the occiput turns a further one-eighth of a circle to the right.

Lateral flexion. The anterior shoulder escapes under the symphysis pubis, the posterior shoulder sweeps the perineum and the body is born by a movement of lateral flexion.

Fig. 21.1: Head descending with increased flexion. Sagittal suture in right oblique diameter of the pelvis.

Fig. 21.2: Occiput and shoulders have rotated one-eighth of a circle forwards. Sagittal suture in transverse diameter of the pelvis.

Fig. 21.3: Occiput and shoulders have rotated two-eighths of a circle forwards. Sagittal suture in the left oblique diameter of the pelvis. The position is right occipitoanterior.

Fig. 21.4: Occiput has rotated three-eighths of a circle forwards. Note the twist in the neck. Sagittal suture in the anteroposterior diameter of the pelvis.

Table 21.1 Terminology for mechanisms of the right occipitoposterior, left mentoanterior and left sacroanterior positions

Possible course and outcomes of labour

Long internal rotation

This is the most common outcome, with good uterine contractions producing flexion and descent of the head so that the occiput rotates forward three-eighths of a circle, as described above.

Short internal rotation (persistent occipitoposterior position)

This results from failure of flexion.

The sinciput reaches the pelvic floor first and rotates forwards.

The occiput goes into the hollow of the sacrum.

The baby is born facing the pubic bone (face to pubis).

Diagnosis

In the first stage of labour:

There is slow descent with a deflexed head and fetal heart audible in the flank or in the midline.

In the second stage of labour:

Delay is common.

On vaginal examination the anterior fontanelle is felt behind the symphysis pubis.

Dilatation of the anus and gaping of the vagina may occur while the fetal head is barely visible; there may also be excessive bulging of the perineum.

The birth

The sinciput will first emerge from under the symphysis pubis; the midwife maintains flexion to prevent it from escaping further than the glabella. The occiput sweeps the perineum and is born. The head is then extended to bring the face down from under the symphysis pubis. An episiotomy may be required to prevent excessive perineal trauma.

Deep transverse arrest

This occurs when the occiput begins to rotate forwards but flexion is not maintained; the occipitofrontal diameter becomes caught at the narrow bispinous diameter of the outlet.

Arrest may be due to weak contractions, a straight sacrum or a narrowed outlet.

Diagnosis

On vaginal examination:

The sagittal suture is in the transverse diameter of the pelvis and both fontanelles are palpable.

The head is at the level of the ischial spines and there is no advance.

Management

Pushing may not resolve the problem, though a change of position and sighing out slowly (SOS) breathing may help to overcome the urge to bear down.

An operative delivery may be required to rotate the head to an occipitoanterior position before birth. Adequate analgesia or anaesthesia should be given.

Conversion to face or brow presentation

This occurs when the head is deflexed at the onset of labour and extension occurs instead of flexion.

Complete extension results in a face presentation; incomplete extension leads to a brow presentation.

Complications

Prolonged labour with increased likelihood of instrumental delivery.

Obstructed labour.

Maternal perineal trauma.

Neonatal trauma/cerebral haemorrhage.

Cord prolapse.

Face presentation

The attitude of the head is one of complete extension. It may be primary (presents before labour) or secondary (develops during labour). The denominator is the mentum and the presenting diameters are the submentobregmatic (9.5 cm) and the bitemporal (8.2 cm) (Figs 21.5–21.10).

Fig. 21.5: Right mentoposterior.

Fig. 21.6: Left mentoposterior.

Fig. 21.7: Right mentolateral.

Fig. 21.8: Left mentolateral.

Fig. 21.9: Right mentoanterior.

Fig. 21.10: Left mentoanterior.

Causes

Anterior obliquity of the uterus.

Contracted pelvis.

Polyhydramnios.

Congenital abnormality, e.g. anencephaly.

Diagnosis during labour

Abdominal palpation

Face presentation may be difficult to diagnose.

The occiput feels prominent, with a groove between head and back.

The limbs may be palpated on the side opposite to the occiput.

The fetal heart is best heard through the fetal chest on the same side as the limbs. It is difficult in a mentoposterior position.

Vaginal examination

The presenting part is high, soft and irregular.

The orbital ridges, eyes, nose and mouth may be felt.

As labour progresses the face becomes oedematous, making it more difficult to distinguish from a breech presentation.

Care must be taken not to injure or infect the eyes.

Mechanism of the left mentoanterior position

See Table 21.1 (p. 232).

Extension. Descent takes place with increasing extension. The mentum becomes the leading part.

Internal rotation of the head. This occurs when the chin reaches the pelvic floor and rotates forwards one-eighth of a circle. The chin escapes under the symphysis pubis.

Flexion. Flexion takes place when the sinciput, vertex and occiput sweep the perineum; the head is born.

Restitution. This occurs when the chin turns one-eighth of a circle to the woman’s left.

Internal rotation of the shoulders. The shoulders enter the pelvis in the left oblique diameter, and the anterior shoulder reaches the pelvic floor first and rotates forwards one-eighth of a circle along the right side of the pelvis.

External rotation of the head. This occurs simultaneously. The chin moves a further one-eighth of a circle to the left.

Lateral flexion. The anterior shoulder escapes under the symphysis pubis, the posterior shoulder sweeps the perineum and the body is born by a movement of lateral flexion.

Possible course and outcomes of labour

Prolonged labour

This is due to an ill-fitting presenting part and facial bones that do not mould. The fetal axis pressure is directed to the chin and the head is extended almost at right angles to the spine, increasing the diameters to be accommodated in the pelvis.

Mentoanterior positions

With good uterine contractions, descent and rotation of the head occur and labour progresses to a spontaneous birth.

Mentoposterior positions

If the head is completely extended and the contractions are effective, the mentum will rotate forwards and the position becomes anterior.

Persistent mentoposterior position

If the head is incompletely extended and the sinciput reaches the pelvic floor first and rotates forwards one-eighth of a circle, bringing the chin into the hollow of the sacrum, no further mechanism is possible and labour becomes obstructed.

Management of labour

See Box 21.2.

Box 21.2 Management of labour: face presentations

First stage of labour

• Inform the obstetrician

• Do not apply a fetal scalp electrode and take care not to infect or injure the eyes during vaginal examinations

• Following rupture of the membranes, exclude cord prolapse

• Observe descent of the head by abdominal palpation

• Assess cervical dilatation and descent of the head by vaginal examination every 2–4 hours. In mentoposterior positions, note whether the mentum is lower than the sinciput. If the head remains high in spite of good contractions, caesarean section is likely

Birth of the head

• When the face appears at the vulva, maintain extension by holding back the sinciput and permitting the mentum to escape under the symphysis pubis before the occiput is allowed to sweep the perineum. In this way the submentovertical diameter (11.5 cm), instead of the mentovertical diameter (13.5 cm), distends the vaginal orifice

• Because the perineum is also distended by the biparietal diameter (9.5 cm), an elective episiotomy may be performed

• If the head does not descend in the second stage and is in a mentoanterior position, it may be possible for the obstetrician to deliver the baby with forceps; when rotation is incomplete, or the position remains mentoposterior, a rotational forceps delivery may be feasible

• If the head has become impacted, or there is any suspicion of disproportion, a caesarean section will be necessary

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Related posts:

1. The Placenta
2. Prolonged Pregnancy and Disorders of Uterine Action
3. The Third Stage of Labour
4. Antenatal Care

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