Malpresentations and malpositions

Usually the fetal head engages in the occipito-anterior position (more often left occipito-anterior (LOA) rather than right) and then undergoes a short rotation to be directly occipito-anterior in the mid-cavity. Malpositions are abnormal positions of the vertex of the fetal head relative to the maternal pelvis. Malpresentations are all presentations of the fetus other than vertex.

Obstetrics - the pelvis and head

Malpresentation

Predisposing factors to malpresentation include:

• Prematurity.

• Multiple pregnancy.

• Abnormalities of the uterus - eg, fibroids.

• Partial septate uterus.

• Abnormal fetus.

• Placenta praevia.

• Primiparity.

Breech presentation

See the separate Breech Presentations article for more detailed discussion.

• Breech presentation is the most common malpresentation, with the majority discovered before labour. Breech presentation is much more common in premature labour.

• Approximately one third are diagnosed during labour when the fetus can be directly palpated through the cervix.

• After 37 weeks, external cephalic version can be attempted whereby an attempt is made to turn the baby manually by manipulating the pregnant mother's abdomen. This reduces the risk of non-cephalic delivery¹ .

• Maternal postural techniques have also been tried but there is insufficient evidence to support these² .

• Many women who have a breech presentation can deliver vaginally. Factors which make this less likely to be successful include³ :

  • Hyperextended neck on ultrasound.

  • High estimated fetal weight (more than 3.8 kg).

  • Low estimated weight (less than tenth centile).

  • Footling presentation.

  • Evidence of antenatal fetal compromise.

Transverse lie⁴

• When the fetus is positioned with the head on one side of the pelvis and the buttocks in the other (transverse lie), vaginal delivery is impossible.

• This requires caesarean section unless it converts or is converted late in pregnancy. The surgeon may be able to rotate the fetus through the wall of the uterus once the abdominal wall has been opened. Otherwise, a transverse uterine incision is needed to gain access to a fetal pole.

• Internal podalic version is no longer attempted.

• Transverse lie is associated with a risk of cord prolapse of up to 20%.

Malposition

Occipito-posterior position

• This is the most common malposition where the head initially engages normally but then the occiput rotates posteriorly rather than anteriorly. 5.2% of deliveries are persistent occipito-posterior⁵ .

• The occipito-posterior position results from a poorly flexed vertex. The anterior fontanelle (four radiating sutures) is felt anteriorly. The posterior fontanelle (three radiating sutures) may also be palpable posteriorly.

• It may occur because of a flat sacrum, poorly flexed head or weak uterine contractions which may not push the head down into the pelvis with sufficient strength to produce correct rotation.

Management

• As occipito-posterior-position pregnancies often result in a long labour, close maternal and fetal monitoring are required. An epidural is often recommended and it is essential that adequate fluids be given to the mother.

• The mother may get the urge to push before full dilatation but this must be discouraged. If the head comes into a face-to-pubis position then vaginal delivery is possible as long as there is a reasonable pelvic size. Otherwise, forceps or caesarean section may be required.

Occipito-transverse position

The head initially engages correctly but fails to rotate and remains in a transverse position.

Alternatives for delivery include manual rotation of fetal head using Kielland's forceps, or delivery using vacuum extraction. This is inappropriate if there is any fetal acidosis because of the risk of cerebral haemorrhage.

Therefore, there must be provision for a failure of forceps delivery to be changed immediately to a caesarean. The trial of forceps is therefore often performed in theatre. Some centres prefer to manage by caesarean section without trial of forceps.

Face presentations

• Face presents for delivery if there is complete extension of the fetal head.

• Face presentation occurs in 1 in 1,000 deliveries⁵ .

• With adequate pelvic size, and rotation of the head to the mento-anterior position, vaginal delivery should be achieved after a long labour.

• Backwards rotation of the head to a mento-posterior position requires a caesarean section.

Brow positions

• The fetal head stays between full extension and full flexion so that the biggest diameter (the mento-vertex) presents.

• Brow presentation occurs in 0.14% of deliveries⁵ .

• Brow presentation is usually only diagnosed once labour is well established.

• The anterior fontanelle and super orbital ridges are palpable on vaginal examination.

• Unless the head flexes, a vaginal delivery is not possible, and a caesarean section is required.