Intended for healthcare professionals

Clinical Review ABC of labour care

Unusual presentations and positions and multiple pregnancy

BMJ 1999; 318 doi: (Published 01 May 1999) Cite this as: BMJ 1999;318:1192
  1. Geoffrey Chamberlain,
  2. Philip Steer

    In the vast majority of deliveries near term the fetus presents by the head, with the best fit into the lower pelvis in the occipito-anterior position. However, although the head is presenting, it may be not in an occipito-anterior but in an occipito-posterior or transverse position. In a few cases the head is grossly deflexed so that the brow or even the face can present.

    In other instances, it is not the head that is at the lower pole of the uterus but the buttocks, or breech (from the old English brec—breeches or buttocks). The fetus many even lie transversely so that no pole is in relation to the pelvic inlet. A fetus in this position is undeliverable vaginally; both transverse lies and breech presentations are much more common if the woman enters labour in the earlier weeks of pregnancy (22-28 weeks of gestation).

    Fetal head engages in left occipito-anterior position (top) then descends into mid-cavity and rotates to full occipito-anterior (bottom)

    All these malpresentations and malpositions need careful diagnosis and skilful management.


    Normal mechanism

    Usually the fetal head engages in the left (less commonly, right) occipito-anterior position and then undergoes a short rotation to be directly occipito-anterior in the mid-cavity.

    If, instead of the normal curve, the sacrum is straightened (shaded area), the anterior-posterior diameter in mid-cavity is reduced (A-A), thus hindering head rotation in this zone

    Occipito-posterior position

    This is the commonest malpresentation. The head engages in the left or right occipito-transverse position, and the occiput rotates posteriorly, rather than into the more favourable occipito-anterior position. The reasons for the malrotation are often unclear. A flat sacrum or a head that is poorly flexed may be responsible; alternatively, poor uterine contractions may not push the head down into the pelvis strongly enough to produce correct rotation; epidural analgesia might sometimes relax the …

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