Niddah 36 - Not Every Blood Makes Woman a Niddah
Not every blood that a woman sees makes her a niddah . In fact, she can become a niddah only in the first seven days of her cycle. Then she can go into a mikveh and be again permitted to her husband. After that begin the eleven days when blood gives her a different status, "zavah". These 7 + 11 day cycles are described here for reference. Today the law is more strict, and she waits for seven days after any blood.
However, before the law changed, if a woman was giving birth not in the days when she become a niddah, then her blood does not automatically make her impure. Rather, if we are sure that this blood is due to childbirth, she remains pure. If, however, she sees blood while giving birth, and then the labor stops, this tells us that the blood was not due to imminent childbirth, and she is ritually impure as a zavah .
How long should the interruption in labor be? Rabbi Eliezer says, twenty-four hours; Rabbi Yehoshua says, a complete day, like Shabbat, from evening till next nightfall. Both derive their law from the same phrase in the Torah. We thus see that a woman can have blood during childbirth, and not acquire any type of ritual impurity. The Sages required her to observe one day of niddah impurity, to prevent possible counting mistakes.
Art: William Redmore Bigg - Birth of the Heir