Labour Stages- Mechanism of Cervical Dilatation

In case of first pregnancy the cervical canal dilates from above downwardsor from the internal os to the external os.So its legth shorts gradually from more than 2 cm to thin rim of few millimeters continuous with the lower uterine segment.This process is called effacement and expressed in procentage so when we say effacement is 70% it means that 70% of the cervical canal has been taken up.
Dilatation of the cervix (external os) starts after complete effacement of the cervix.

In case of past delivery effacement and dilatation occur simultaneously.

In normal presentation and position,the head is appled well to the lower uterine segment dividing the amniotic sac by the girdle of contact into hindwaters above it containing the foetus and a forewaters below it.This reduces the pressure in the forewaters preventing early rupture of membranes.After full dilatation of the cervix the hind and forewaters become one sac with increased pressure in the bag of forewaters leading to its rupture.

Phases of cervical dilatation:
1.Latent phase:
-this is the first 3 cm of cervical dilatation which is slow takes about 8 hours first pregnancy and 4 hours in multiparae
2.Active phase:
It had 3 components:
-acceleration phase
-maximum slope
-deceleration phase.
The phase of maximum slop is the most detectable and two other phases are of shorter duration and can be detected only by frequent vaginal examination.
The normal rate of cervical dilatation in active phase is 1.2 cm/hour in case of first pregnancy and 1.5 cm/hour in case of 2nd opregnancy.If the rate is less than 1 cm/hour it is considered prologed.