According to the ruling of R’ Yehuda Henkin ztz”l, the founding posek of this site, a woman becomes niddah during childbirth either when she begins bleeding, is no longer able to walk unaided due to the strength of the contractions, or is fully dilated (10 cm).
1) If a woman in labor is still able to walk about unaided, and is not yet experiencing bleeding, she is not yet niddah, even if she is partially (but not fully) dilated. A woman in childbirth is not obligated to be examined to determine how dilated she is.
2) On its own, the water breaking does not render a woman in labor niddah if it is not accompanied by bleeding. See our posted question on this topic for more details.
3) When the mucus plug is expelled, the woman does not become niddah unless it was accompanied by bleeding. According to some opinions, this would be true even if the mucus plug itself contained some blood, since that blood is not from the uterus.