Intimacy is forbidden to a married couple while the woman is a niddah, on the basis of the words of the Torah, which we have quoted more than once above:
“And you shall not approach a woman during her separation (niddah), in her impurity, to uncover her nakedness”[1].
The root of the word “niddah” is “nod.” It means “to move away,” “to move,” “to wander.” Understood in this sense, the word “niddah” describes the body's condition and the relationship between spouses during this period.
During this time, the lining of a woman's uterus breaks down and, together with blood, “moves”—is expelled—from the body. During this period, the spouses are not one whole but are separate, “moved apart” from one another. Yet they remain bound by emotional closeness; their “moving apart” is temporary. After a short time, following immersion in a kosher mikveh, everything will fall back into place. That is precisely why, to describe the status of a woman during her menstrual period, the Torah uses a special term —“niddah,” from the word “nedidah”—“temporary movement”—and does not apply to it the negative terminology (such as “abomination”) that describes other forbidden sexual relationships.
The purifying waters of the mikveh are called by the Torah[2] “waters of niddah.” Notice that the very same name, “waters of niddah”[3], or “waters of nedidah,” is given to a mixture of water and the ashes of the red heifer, the most powerful “purifying agent” the Torah gives us. The Torah prescribes using this mixture to purify a person who has become “impure” through contact with a dead body. (We remind you once again: the concept of “impurity” in this context has nothing to do with physical dirt—it refers to a spiritual phenomenon.)
[1] Vayikra, 18:19
[2] Bamidbar, 31:23. Bamidbar (“In the Wilderness”) is the fourth book of the Pentateuch.
[3] Bamidbar, 19.