Power, passenger and passage: the Three Ps of slow labour
The Three Ps are power, passenger and passage: a checklist for working out why a labour is slow or difficult.
Remember
When labour is slow, sort the cause into power (the contractions), passenger (the baby) or passage (the pelvis and the soft tissues the baby must travel through). The Three Ps guide a full clinical assessment rather than replace it.
When a labour is slow or difficult, clinicians sort the possible causes into the classical three categories: power, passenger and passage. A problem with any one of them can hold up labour, so the clinician asks about each in turn.
Power is the force that drives labour: the contractions of the muscle of the uterus, judged by how strong and how frequent they are. In the first stage of labour, which runs from the onset of labour to full dilation (opening) of the cervix (part of the birth canal), contractions ideally come every 3 to 4 minutes. If they are too weak or too infrequent, an intravenous infusion of oxytocin (the hormone that makes the uterus contract) can be given to strengthen them. The mother's pushing also belongs to power, adding to the force made by the contractions.
The passenger is the baby: its size, its lie (the direction of its long axis compared with the mother's), its presentation (the part that lies lowest and comes out first), its position (for example occipitoposterior, with the back of the head towards the mother's back), the flexion of its neck and the moulding of its skull (the overlap of the skull bones at their sutures). A head that is poorly flexed, meaning the chin is not tucked onto the chest, or that lies occipitoposterior, presents a larger diameter and can slow labour.
The passage is the birth canal the baby must travel through: the maternal pelvis and the soft tissues, including the cervix.
The three also interact, so one finding does not prove the cause. A large baby causes difficulty mainly when it is too big for the pelvis, a mismatch called foetopelvic disproportion (the foetus cannot fit through the pelvis). The framework organises the assessment; it is not a diagnosis.
Key facts
- Power
- contractions that are too weak or too infrequent can slow labour.
- Passenger
- the baby's size, lie, presentation, position, neck flexion and head moulding.
- Passage
- the maternal pelvis and the soft-tissue birth canal, including the cervix.
- Smallest head diameter
- an occipitoanterior head (the ideal position for delivery) with the neck well flexed (chin on chest) gives the smallest head diameter.
- Interaction
- the three work together, so a large baby and a small pelvis combine to cause disproportion.
- Not among the Three Ps
- the placenta, pain and pressure.
Exam traps
- Strong, regular contractions do not point to a power problem, because poor power would usually show as weak or infrequent contractions.
- Ineffective maternal pushing belongs to power, not to the passenger, because it adds to the expulsive force made by the contractions and does not reflect the baby's size, lie, presentation or position.
- A scarred cervix that opens slowly is a passage problem, not a passenger problem, because the cervix is part of the birth canal and the baby itself is not the cause.
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