Essential factors of labor birth canal
This powerful segment draws the weaker, thinner and more passive lower segment up over its contents, and in so doing pulls up and then dilates the cervix.
The wall of the upper segment becomes progressively thicker with progressive thinning of the lower segment. This is pronounced in late first stage, specially after rupture of the membranes and attains its maximum in second stage.
A distinct ridge is produced at the junction of the two segments, called physiological retracting ring.