After a woman gives birth, she is considered Teme’a (ritually impure) like a Nidda. This status extends for seven days after the birth of a boy, and for fourteen days after the birth of a girl. After this period of seven or fourteen days, she begins a period called "Yemeh Tohar," during which bleeding does not render her a Nidda. In the case of the birth of a boy, this period extends for thirty-three days after the original seven days; in the case of a girl, this period extends for sixty-six days after the fourteen days in which she is considered a Nidda.
Therefore, according to Torah law, if a woman was able to immerse and become Tehora (ritually pure) after the completion of seven days after delivering a baby boy, she remains Tehora for the next thirty-three days even if she experiences bleeding. Likewise, if she was able to immerse two weeks after delivering a girl, she cannot, according to Torah law, become a Nidda for the next sixty-six days, even if she experiences bleeding. Of course, if she did not immerse after the original seven or fourteen days, then she is in any event a Nidda, irrespective of any blood sighted during the period of "Yemeh Tohar."
However, the Ge’onim (Rabbis of the post-Talmudic period) enacted that we should consider a woman a Nidda if she experiences bleeding during the "Yemeh Tohar." Therefore, a woman who experiences bleeding during these weeks must observe all the restrictions of Nidda, even though she had immersed after the original seven or fourteen days, and despite the fact that she is in the middle of the "Yemeh Tohar." She remains a Nidda until she undergoes the usual process of waiting five days, performing the Hefsek Tahara and Moch Dahuk inspections, observing the "seven clean days," and, finally, immersing in a Mikveh.
There is, however, one difference between a woman who experiences bleeding during the "Yemeh Tohar" and a woman who experiences bleeding under normal circumstances. This difference relates to the status of a woman who notices a "Ketem" – a bloodstain. If a woman does not feel a menstrual flow, but notices a bloodstain on her garment, she is not considered a Nidda according to Torah law, but the Sages enacted that she must nevertheless observe the laws of Nidda. If the stain is discovered on a white garment and is the size of a Geris (approximately the size of the old subway tokens), and a Rabbi has determined its color to be the color of a Nidda stain, she must observe all the laws of Nidda.
However, if this occurs during the "Yemeh Tohar," then her status is somewhat more lenient. If a woman during the "Yemeh Tohar" discovers a "Ketem" which renders her a Nidda, then she does not have to wait five days before beginning the "seven clean days." Consider the case of a woman who gives birth to a girl, and two weeks later is able to immerse in the Mikveh and become Tehora. Then, two weeks after her immersion, she discovers a bloodstain and brings it to a Rabbi, who determines that it indeed qualifies as a "Ketem" and renders her a Nidda. In such a case, already that day she should thoroughly wash the area and perform the Hefsek Tahara and Moch Dahuk inspections. She should then observe the "seven clean days" and immerse in the Mikveh the night after the seventh day. Since the status of Nidda during the period of "Yemeh Tohar" applies only on the level of accepted custom, and not on the level of Torah law, we treat situations of a "Ketem" more leniently, and allow the woman to begin the "seven clean days" immediately. This is the ruling of Hacham Ovadia Yosef. It should be noted that this marks the only situation where a woman who is Tehora and then becomes a Nidda can begin the "seven clean days" without waiting five days.
Summary: A woman is considered a Nidda after childbirth for a period of seven days in the case of a baby boy, and fourteen days after the birth of a girl. After this period, if she is able to establish the cessation of bleeding, she may immerse in a Mikveh and become Tehora (ritually pure). In such a case, if she experiences bleeding during the thirty-three days after the original seven days (in the case of a boy), or during the sixty-six days after the original fourteen days (in the case of a girl), she is not a Nidda according to Torah law, but is nevertheless considered a Nidda by force of accepted custom. She must observe all the ordinary laws of Nidda during this period of thirty-three or sixty-six days, except that in the case of a bloodstain, she may begin the "seven clean days" immediately, and does not have to first wait five days as is normally required.
