Halacha is a dynamic process in which rabbinic rulings are debated. Some opinions are eventually accepted while others are rejected, and there are sometimes local differences regarding accepted practice. Therefore, variations exist between the US and Israel, and even between different communities within the same country.
As to the particular question of rupture of the amniotic membranes (“water breaking”), our rabbinic supervisor, Rav Yehuda Henkin, rules that a woman in childbirth becomes niddah when she either experiences bleeding, reaches the point at which the contractions are so strong she can not walk unaided, or is fully dilated. The water breaking during this part of active labor would be additional proof of the fact that she is niddah. On the other hand, if she has premature rupture of the membranes (without bleeding) prior to active labor this would not make her niddah.
If her membranes are artificially ruptured by the physician, the halacha would depend on the size of the instrument used for the procedure. Opinions as to this size vary. Rav Henkin rules that an instrument with a diameter of 19 mm or more would make a woman niddah.