Fetal malpresentation exists when the presenting part is other than the normal vertex of the fetal head. This includes two malpresentations that are covered in other chapters: breech ( Chapter 16 ) and cord presentation ( Chapter 18 ). The remaining malpresentations that will be covered in this chapter are face, brow, transverse lie with shoulder or arm presentation, and compound presentations. In modern obstetrics, particularly in the developed world, the incidence of malpresentations has fallen. This is due to the association of many malpresentations with high parity and the fact that women are having fewer children.
The various anteroposterior diameters ( Fig 11-1 ) of the term fetal head vary depending upon the position: normal flexed vertex (9.5 cm), deflexed occipito-posterior position (11–12 cm) and malpresentation: face presentation, submento-bregmatic (9.5 cm) and brow, mento-vertical (13.5 cm). These are illustrated in Figure 11-2 .
Face Presentation
In face presentation the attitude of the fetal head is one of complete extension with the chin as the denominator and leading pole. The presenting diameter is the submento-bregmatic, which in the term fetus is about 9.5 cm. This is the same as the favourable flexed vertex presentation, but the facial bones do not mould to the same extent as the cranial vault does in vertex presentation. The incidence of face presentation is about 1 in 500 births.
Causes
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Fetal anomalies are found in about 15% of face presentations. The commonest are major CNS anomalies such as anencephaly and meningomyelocoele. Tumours of the neck may also cause extension and face presentation.
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Prematurity.
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Mild cephalopelvic disproportion has been incriminated. It is probable that in some cases of deflexed occipito-posterior position with relative disproportion the fetal head may extend completely to a face presentation.
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It has been postulated that excessive tone in the extensor muscles may predispose to face presentation. This theory has been used to explain cases of primary face presentation which occur before the onset of labour. The development of face presentation during labour is called secondary face presentation.
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High parity is associated in the majority of cases.
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In most cases, other than parity, no obvious cause is found.
Diagnosis
The presenting part of the face is between the chin and supraorbital ridges. Usually the characteristic landmarks of the eyes, nose, mouth and chin can be felt with the examining finger during labour. Considerable oedema often develops which may, to a degree, obscure these landmarks. Although the distinction is usually obvious there may be confusion in distinguishing between the mouth and the anus. If this is so, the finger is inserted into the orifice and the gum ridges can easily be felt as a distinguishing landmark.
Diagnosis before labour is rare but may be suspected if on abdominal examination the fetus is easily palpated and its back is lying dorsoanterior. In cases with a normally flexed vertex, palpation of the back and head will reveal only a slight depression at the neck between the back and occiput. In face presentations on the other hand, with the head extended, there is a marked depression between the back and occiput. If there is clinical suspicion, ultrasound will establish or refute the diagnosis.
The position of a face presentation is defined with the chin as the denominator and is therefore recorded as mento-anterior, mento-posterior or mento-transverse, left or right accordingly. The majority of cases are mento-anterior.
Management
On the rare occasions that face presentation is diagnosed before labour a careful ultrasound examination should be made to exclude structural fetal anomalies. One can make the case for observation until the onset of labour or full term, on the grounds that a number of these cases will revert spontaneously to a normal flexed vertex position. However, if face presentation persists and the fetus is normal, it should be delivered by elective caesarean section.
When the diagnosis is made in labour a gross fetal anomaly should be excluded and clinical pelvimetry should rule out obvious pelvic contraction or deformity. The position of the face presentation is then assessed. Depending upon the estimated fetal weight, the position, station, clinical assessment of pelvic capacity and the progress of labour the following guidelines may be used:
If the fetus has an anomaly incompatible with life then progress to vaginal delivery should be followed.
If the position is mento-anterior, which presents the same diameters as a flexed vertex, if the fetus is normal or small in size and the pelvis is of good capacity, progress can be followed with the expectation of spontaneous vaginal delivery. The majority of cases with mento-transverse position will rotate to the more favourable anterior position.
Fifty years ago, when the morbidity and mortality associated with caesarean section was high, vaginal manipulation was used to try and convert face presentations to a vertex presentation. This was carried out at advanced or full cervical dilatation and usually done under deep general anaesthesia with uterine relaxation. In a previous edition of this book Chassar Moir (1964) described his technique:
‘In cases discovered early in labour (mentolateral) positions, I succeeded in five consecutive cases in correcting the position to a vertex position by the simple intrauterine manipulation of hooking down the occiput with the fingers and simultaneously pressing up the chin and brow with the thumb, labour then proceeded normally in each case.’
Nowadays, we would not advise such manipulation, except perhaps a tentative attempt which is most likely to succeed with a small fetus and a large pelvis. One would only continue with such a manoeuvre if it proved easy and atraumatic.
‘When the chin is turned towards the pubis, at the lower part of that bone, the woman must be laid on her back, the forceps introduced … and when the chin is brought out from under the pubis, the head must be pulled half round upwards; by which means, the fore and hind head will be raised from the perineum.’
William Smellie
A Treatise on the Theory and Practice of Midwifery. London: D. Wilson, 1752, p281
Provided labour continues normally and there is good progress in the second stage with mento-anterior positions, spontaneous delivery is likely.
If progress is inadequate with mento-anterior positions consideration of forceps delivery is appropriate. One has to be very careful that there is adequate descent for safe forceps assisted delivery. Even when the face is visible at the vulva the cranium may not be fully through the pelvic brim. The guiding dictum is ‘the head is higher than you think’. If forceps delivery is to be considered there should be no head palpable at the pelvic brim and the sacral hollow should be filled with the cranium. Either classical or Kielland’s forceps can be used and in face presentations the chin replaces the occiput for orientation. If Kielland’s forceps are used the directional buttons on the shanks should point toward the chin. With both types of forceps the blades are applied as for classical forceps to the occiput anterior positions (see Chapter 10 ). The orientation is along the mento-occipital diameter of the head. With the pelvic curve of the classical forceps the chin is between the heels of the blades and the face is beneath the level of the shanks ( Fig 11-3 ). With Kielland’s forceps the upper part of the blades is at the level of the supraorbital ridges with the face above the level of the shanks ( Fig 11-4 ).
