Three mechanical factors determine progress during labour.
The powers – the degree of force expelling the fetus
In established labour, the uterus contracts for 45–60 seconds every 2–3 minutes. This pushes the fetal head into the pelvis and causes effacement and dilatation. Incoordinate uterine activity is commonly seen in nulliparous labours but is rare in multiparous women.
The passage – the dimensions of the pelvis
The bony pelvis has three principle planes with varying diameters. The fetal head rotates to negotiate the pelvis as it descends. Cervical dilatation is a prerequisite for delivery. In the second stage the soft tissues of the vagina and perineum stretch or tear to allow passage of the head.
The passenger – the diameters of the fetal head
Several factors determine how easily the fetal head fits through the pelvis:
- the attitude (degree of flexion)
- the position (degree of rotation)
- the size of the head.