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'.

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

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

PassengerPassage

Head:

● Large fetal head (big for that pelvis)

● Hydrocephalus (brain surrounded by fluid, which makes the skull swell)

Presentation and position:

● Brow, face, shoulder

● Persistent malposition

Twin pregnancy:

● Locked twins (locked at the neck)

● Conjoined twins (fused together with some shared organs)

Bony pelvis:

● Contracted (due to malnutrition)

● Deformed (due to trauma, polio)

Soft tissue:

● Tumour in the pelvis

● Viral infection in the uterus or abdomen

● Scars (from female circumcision)

The mechanical causes of prolonged and obstructed labour shown in the table above 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.