As indicated above, obstructed labour is generally a second stage phenomenon, in women whose labour is prolonged. Why labour becomes prolonged or obstructed may be due to one of the 'Ps' (as midwives and obstetricians call them): 'powers', 'passenger' and 'passage'.

• Powers: Inadequate power, due to poor or uncoordinated uterine contractions, is a major cause of prolonged labour. Either the uterine contractions are not strong enough to efface and dilate the cervix in the first stage of labour, or the muscular effort of the uterus is insufficient to push the baby down the birth canal during the second stage.
• Passenger: The fetus is the 'passenger' travelling down the birth canal. Prolonged labour may occur if the fetal head is too large to pass through the mother's pelvis, or the fetal presentation is abnormal.
• Passage: The birth canal is the passage, so labour may be prolonged if the mother's pelvis is too small for the baby to pass through or the pelvis has an abnormal shape, or if there is a tumour or other physical obstruction in the pelvis.

Table 9.1 summarises the mechanical causes of 'passenger' and 'passage' failure.

Table 9.1 Causes of passenger and passage failures that lead to prolonged and possibly obstructed labours.

The mechanical causes of prolonged and obstructed labour shown in Table 9.1 can be grouped into various categories: cephalopelvic disproportion; malpresentations and malpositions; or an abnormality in the fetus or the mother which obstructs the birth canal. We will look at each of these in turn in more detail.