At the end of every article you'll find a summary of the practical halachah. If this isn't the moment to go deep, skip straight to the end.

We study the laws according to the order of the Shulchan Aruch, which devotes 17 simanim to the laws of niddah. The first is Siman 183, and that is where we begin. Siman 183 contains little practical halachah, but it is the foundation of everything that follows.

Beginning at the beginning — the verses

Before we reach the Shulchan Aruch, we start with the Torah, which describes two kinds of blood.

Dam niddah

"And you shall not come near a woman in her time of unclean separation, to uncover her nakedness." (ויקרא יח, יט)

Dam zivah

"And when she is cleansed of her discharge, she shall count for herself seven days, and after that she shall be pure." (ויקרא טו, כח)

The difference between a niddah and a zavah

The essential difference is the timing of the bleeding. By Torah law, a woman who bleeds during the days that are her days of niddah is a niddah; a woman who bleeds during the days that are her days of zivah is a zavah.

Depending on how long she bleeds during the days of zivah, she is either a zavah ketanah or a zavah gedolah, and each status carries different consequences. The poskim disagree about how these days are calculated, and that disagreement is one of the reasons we make no attempt today to calculate days of niddah or days of zivah.

The practical difference

By Torah law:

The chumra of Rabbi Zeira

Today every woman who experiences uterine bleeding (a stain is a separate subject) conducts herself as a zavah. This is the chumra of Rabbi Zeira — which Rabbi Zeira did not in fact initiate. The righteous women of that generation took the stringency upon themselves, and Rabbi Zeira affirmed it.

Why did those women choose a stringency?

Was it only that they were so pious that an added stringency cost them nothing, even one that affects mitzvat onah, peru urvu and shalom bayit? Not quite.

The reason this stringency was necessary is worth understanding, because some women today want to "keep niddah on the Torah level" — to set aside the chumra of Rabbi Zeira and immerse seven days from the onset of bleeding without counting seven clean days. As we will see, keeping niddah on the Torah level without the chumra of Rabbi Zeira is not possible.

First reason — calculating the days

As mentioned, the poskim disagree about how to calculate days of niddah against days of zivah. That alone leaves us unable to know whether a given woman is a zavah or a niddah.

Second reason — the color of the blood

There is a further concern: the color of the blood. The Mishnah teaches that only four shades of red, and black, render a woman temei'ah. The Gemara records testimony from various sages, among them Rabbi Zeira and Ulla, who refused to examine bedikah cloths because they could not reliably distinguish the pure shades of red from the impure ones. They simply could not tell a woman whether her bedikah was clean.

The Rosh teaches that in our generations no sage knows how to distinguish pure shades of red from impure ones. Any color carrying even a hint of red is therefore treated as impure, out of doubt that it may be one of the four impure shades.

How this connects to the chumra of Rabbi Zeira

Suppose a woman decided to "keep niddah on the Torah level" even though she has no way of knowing whether she is seeing dam zivah or dam niddah. Suppose further that it is in fact dam niddah. She bleeds for six days and assumes that on the seventh she may immerse under the Torah law of niddah.

But what if the blood of the first five days was a pure shade, and only on the sixth day did the color turn impure? Then day six was the real onset of her days of niddah, and another seven days must pass before she may immerse.

Why the chumra of Rabbi Zeira cannot be waived

Now we can see why the chumra of Rabbi Zeira is not negotiable. That is quite apart from the fact that rabbinic mitzvot are full mitzvot in their own right. But it explains why, unlike other rabbinic obligations, this one is not set aside in the face of a Torah obligation.

An example: a woman with halachic infertility, whose ovulation falls before the mikveh night. She and her husband cannot fulfill peru urvu. We still cannot permit her to immerse after seven days as a Torah-level niddah. She must count seven clean days under the chumra of Rabbi Zeira. (Other routes to resolving halachic infertility are of course explored.)

The ruling of the Shulchan Aruch

The Shulchan Aruch draws no distinction between niddah and zavah, because it records only the practical halachah: a woman who experiences uterine bleeding must perform a hefsek taharah and count seven clean days before immersing in a kosher mikveh.

Practical halachah

A woman who experiences uterine bleeding accompanied by a hargashah (see the article on hargashah), even in a quantity as small as a mustard seed, must perform a hefsek taharah and count seven clean days before immersing in a kosher mikveh.