Obstructed labour is the failure of the fetus to descend through the birth canal, because there is an impossible barrier (obstruction) preventing its descent despite strong uterine contractions.

The obstruction usually occurs at the pelvic brim, but occasionally it may occur in the pelvic cavity or at the outlet of the pelvis. When labour is prolonged because of failure to progress, there is a high risk that the descent of the fetus will become obstructed.

There is no single definition of prolonged labour, because what counts as 'too long' varies with the stage of labour.

When is labour classed as prolonged in the different stages of labour?

Prolonged latent phase of labour: when true labour lasts for more than about 8 hours without entering into the active first stage.
Prolonged active phase of labour: when true labour takes more than about 12 hours without entering into the second stage.
Prolonged second stage of labour:
- Multigravida mother: when it lasts for more than 1 hour.
- Primigravida mother: when it lasts for more than 2 hours.

Although labour can be classed as 'prolonged' at any stage, you should note that obstructed labour most commonly develops after the labour has entered into the second stage.