The Onset of the Niddah Status in Labor
A woman becomes a niddah when her cervix is dilated during the final stages of labor. She is not halachically required to undergo an internal examination to determine the degree of dilation. From the point that she can no longer walk unaided, we assume that her uterus has opened enough to render her a niddah.
Uterine bleeding during labor also renders a woman a niddah. Rabbinic authorities differ about the halachic significance of the water breaking, and of the release of the mucus plug.
If a woman's cervix is dilated by a few centimeters early in labor (or earlier in her pregnancy), but she has not had any bleeding, she does not yet become a niddah.
A woman enters the additional status of yoledet (a woman who gives birth) with the vaginal birth of baby. The laws that apply to a yoledet are basically identical to those for a niddah. Therefore, since she already became niddah during labor, this has little practical significance. (A yoledet may not immerse until 14 days have passed since the birth of a girl, but postpartum bleeding rarely ends soon enough for this to be possible.)
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